Koss Pro4S Full Size Studio Headphones, Black with Silver Trim - Reviews
Pros: Very comfortable, sturdy, cable can be plugged in on either side, sound is musical, bass is strong, good seal, comfortable
Cons: Cable is easily yanked out, soundstage not huge
Let me first say that I've had a Sennheiser Momentum 1.0 and a Shure SRH440 with the 840 earpad mod, and I've sold BOTH because they didn't stack up to the Koss Pro4S.
What's so surprising about that is that I didn't even intend to get these, they were a gift. I'd had Koss phones before and really liked them, but didn't know anything about this model. Both those cans, which are much more acclaimed, felt plastic and brittle and lacked a present low end. I'm not even a bass-head but something in those was just sterile and lacking. These aren't monitoring headphones, they're definitely pleasantly colored in a way that sounds natural.
The highs might be a tad recessed for some people, which was apparent when comparing to the SRH440s that are much brighter but have a kind of honky high-mid. I like this though because at louder volumes the highs tend to jump out more, so when cranked the sound evens out and isn't harsh.
The only real universal problem would be that the cable can be easily yanked out since it doesn't twist-lock; it's simply a regular coiled 1/8th inch. It's great that it can be plugged in to either side, a super convenient feature I'm surprised isn't more common.
They're sealed but not dome-like, so they do rest on the ear, but are somehow surprisingly comfortable, even moreso than the SRH440 which just about cover the ear entirely but press against it. I'd probably say it's because the pads on the Pro4S, while pleather, are very soft and dampening. They make for a good seal and I've worn these for 6+ hours with pretty much no fatigue.
They come with a small case and are collapsible/foldable. Also people might not know that they can be daisy chained - if you have two pairs, you can connect them via 1/8th inch and they'll both play the input going in to one of them.
You could maybe do better for the budget, but I'm more than happy with them. I use them with my iPod and laptop and think they're great. For a portable sealed can I think they're hugely overlooked and surprised they aren't more popular. I expected the Momentum and SRH440 to blow them away but since the comparisons my opinion of them has greatly risen.
  • Like
Reactions: stalepie
Pros: Comfort, Build Quality, Lifetime Warranty, Isolation, Warm detailed sound
Cons: Compressed soundstage
I purchased these headphones as a portable/casual listening alternative to my AKG K702 set. My parameters were something comfortable, easily transportable, well constructed and durable, while retaining as much sound quality and detail as possible. I did not want to be limited to the necessity of a portable amp and my budget was no more than $200. I primarily listen to all kinds of rock and heavy metal from every decade, EDM, hip hop and classical. I have also used the PRODJ100 set frequently for casual listening at work, so I will use them for comparison; they have many similarities to the Pro4s.
The construction of these headphones is excellent. They have lovely metal cups and the ear cushions (leather or PU, not sure) are much nicer than the PRODJ100's vinyl which cracks and peels over time. The ear cushions also have an very comfortable "memory foam" padding, which helps them seal around your ears much nicer. The D-shape cup design was not a selling point for me, but it make it easy to differentiate left from right quicker and they are properly comfortable. The dual choice entry cord was not a selling point for me either, but I will say that it is very nice to have the option. When stored in the included, high-quality case, they lay flat and the overall case is not very large. This case is a fantastic inclusion and saved me the time and effort of finding a third-party case online.
When researching this purchase, closed-back/semi-open/open back was not a consideration. These being closed back and having nice ear cushions, block out extraneous noise quite well. I would say that they are as easy to amplify as the PRODJ100; they strain my Samsung Galaxy S3 at high listening levels, but with my MacBook Pro or iMac, half-way on the system volume is quite loud. Now for the sound!
The low end is punchy, quick and full and it extends quite low. These headphones leave little to be desired in the low end, in my opinion. Heavy metal double kick, DnB's growling reese and bass stabs, old-school boom-bap east coast to trap beats: it all sounds rich and fitting for the styles. If anything, I would say that it is slightly exaggerated bass response. The midrange is warm and full. Vocals have a velvety texture and overdrive guitar has a lovely presence, balanced by the lower-midrange fullness. The upper-midrange and treble shows much more detail than the PRODJ100s, yet it does not sound cold or artificial.
The weakest aspect of these headphones I perceive to be the soundstage. Stereo guitars in a dense metal mix jump out, but snare or vocals can easily fall underneath, depending on the quality of the mixing. Some mixes sound great, with others I find myself searching for that crack of the snare or the intelligibility of a lead vocal. I have really only seen this as an issue with a very dense and compressed mix, such as what is common to contemporary heavy metal.
Even with the more narrow soundstage, I still enjoy these headphones. I would not use them to mix my own tracks and I would never call these headphones analytical, but they are certainly an enjoyable pair to listen to, and that is precisely what I purchased them for. They are warm, forgiving and rich sounding closed cans with punchy bass and a fair amount of detail and for that, they are a great value.
Definitely sounds like one to consider. They look like they're well made. I assume with studio monitors the point is to hear single tracks clearly, such as vocals or a single instrument, which is why they lack soundstage. 
Personally I think some of the soundstage issue is how close they are to the ears. If they had bigger pads and cups, I think the soundstage would be improved, but it's clear that Koss designed these to be sleek and lightweight.
I've been using them regularly since my review and they are nice mixing/engineering companions with my AKG K702s, as well as an excellent "portable listening" option.
Pros: Detail/clarity, warm and smooth sound, punchy and tight bass
Cons: Soundstage is lacking
I just got these in from Koss today and initial impressions are incredibly positive. These are Koss' second best headphone available that I have tried (DJ100, DJ200, PRO4AA, PRO4AAT, PRO4AAAT, and MV1). The overall tone of these is warm, detailed, and bassy. The tone is in ways similar to the DJ100, with them being bassy and warm, but these are more detailed and airy. They are more like a DT990 junior, but don't have the dip in the mids and have a smaller soundstage. They honestly stomp the DJ100 into the ground from my testing. 
Detail and resolution is quite good, much better than the DJ100 and even their older MV1 (which wasn't all that great a headphone, ouch!). Right now I am comparing them side by side with the KRK KNS8400, and they are on par when it comes to detail retrieval and transparency, but have added warmth and much more bass.
Treble is really nice on this headphone, and while I would consider it bright, it is never strident or sibilant. The treble is quite smooth and articulated, something that is noteworthy for this headphone considering the price. Extension is quite good but rolls off at the upper most registers. 
The midrange is surprisingly not lacking like the PRO3AAT, but not quite as full bodied as the DJ100. They are noticeably warmer and forgiving than the KRK KNS8400. But don't let that make you think they aren't as detailed, they sure are. I noticed the lower mids blend in nicely with the bass and never seems muddy or out of place. At least they aren't like the Fidelio L1's lower mids where they bleed into the upper mids.
Oh yeah the bass, it thumps a ton! Not quite DT770 levels, but a ton more controlled with good extension. Still quite pronounced, at least as much bass as the Beyerdynamic Custom One Pro on the third adjustment notch. Still very well controlled and never feels bloated, even with bass heavy music like Dubstep and Rap. Quite a feat indeed.
My main gripe is their soundstage isn't that big. It's rather small, perhaps smaller than the DJ100 and most certainly smaller than the KRK KNS8400. But it's not terrible or flat, just more narrow. This gives these headphone a more Grado-esque presentation, as they are a more forward headphone with the treble and bass. 
Overall I am incredibly surprised by these. I'd wager that they are better than the KRK KNS8400. These however are not "studio" headphones. They are not neutral as Koss marketed them. They are warm and bassy. But now the question is how do they compare to the ATH-M50? Well I don't have one to compare to but these are easily the best under $200, perhaps more than that. I really, really like them, and I rarely like stuff from Koss.

You said 2nd best...  For comparison, what do you consider to be Koss' Best Headphones?  I only know their Porta Pros, so I find this headphone interesting as a travel (Hotel rooms) alternative to the Porta Pros for extended stay trips.
I consider the ESP950 their best. It's an electrostatic headphone but don't let that discourage you. It sounds really good. It's only downfall is that the included amp is poop.
If you liked the bass and warmth of the PortaPro, you'll like this. This has a similar signature but is brighter. The soundstage is also much bigger. 
Pros: Clean, Rich and Smooth sound. Pleasurable even after many hours + Detachable cord with dual choice entry.
Cons: Limited soundstage (can be a plus for mixing), Tight fitting (at first), Weird packaging...
I recently ordered the Koss Pro4s and PortaPros to add to my collection. Both of them arrived today (3 days for shipping) and immediately I ran into some quite difficult packaging. The covering over the box had been glued to the box, making it extremely difficult to preserve the outer covering when trying to open it. After managing to cut the cover off and open the inner box, the user is greeted with a nice intro letter and signature from the chairman of Koss. The headphones come packed in a very nice carrying case (like the PRODJ200s) and with a nice coiled cable + a 1/4" adapter.
Design/Build Quality/Functionality:
These headphones look and feel really good and quite durable. The construction seems very solid and they should last a very long time. They do clamp a bit tight at first but its nothing I can't get used to and the cushions are very comfortable. They definitely isolate better than the PRODJ200s do. One of the main things I like about these headphones is that the cable can be plugged into either side, and the other jack can be used as an output to other headphones which is an awesome feature to have, especially in studio environments. I can definitely see these becoming popular in professional environments and even for casual listening. They also fold flat for transport in the included carrying case.
Out of the box, these headphones have quite a bit of low-end bass (quite a bit more bass than my well worn PRO-DJ200s do and THOSE were advertised as "extreme bass headphones"), but the bass of the Pro4s is definitely quick and punchy. As of right now, my PRO-DJ200s sound more balanced. I'm guessing the bass of the Pro4s will eventually quiet down after several hours of burn in (much like the PRODJ200s did.) One slight benefit of the PRODJ200 when it comes to sound is the slightly wider soundstage. But there is definite gain in clarity with upper-mids and treble when going from the PRODJ200 to the Pro4s, even though the mids may not be quite as forward as they are in the PRODJ200s. The Pro4s can definitely handle loud volumes without much distortion and they feel and sound very solid. I've been listening to all kinds of music (even as I write this review.) The more I hear these things, the more I fall in love with them, and It will be my pleasure to put some miles on these and provide updates as the burn-in begins to settle.
Here are some albums I have played through these so far:
(Mostly CD or Flac/MP3 @ at least 320kbps, Through my computer or CD player, and into a vintage NIKKO NR-819 receiver.)
Tears For Fears - Everybody Loves A Happy Ending - One of my favorites for overall quality and good mastering, and it sounds great in these headphones.
My Chemical Romance - The Black Parade (I love how the Drums really stand out + crisp and crunchy guitar tones.)
Lake Street Dive - Bad Self Portraits - Very good vocal clarity... Quite a bit of bass (still burning them in and the recording is pretty bassy) so hopefully it will balance out eventually.
Trigger Hippy - Trigger Hippy (2014) - Awesome guitar tones and very nice mid-treble detail especially with vocals.
Yes - The Yes Album - The 60s/70s - style panning and mixing makes for a fun listening experience.
Sound City - Reel To Reel - Nice punchy bass and crunchy/sharp guitar tones.
Pink Floyd - Dark Side of The Moon (Always a good album for testing audio systems) - Again, The 60s/70s - style panning and mixing on this type of album sounds really good in these headphones.
+ DVD Concerts including; Frank Zappa -  A Token of His Extreme and Steve Winwood - Live In Concert (2005?)
Excellent and spot on review.