The ZS4 are the best bang for buck of all of my KZ. I am really enjoying them, and to think they only cost 12 USD is astonishing!
I used them today in my 30 min commute on a hybrid tram-subway that grinds the rails like ork's nails running down a chalkboard when it gets to the surface. Just so you know, when I use my PortaPros, I suddenly stop listening to whatever in those situations. Not with the ZS4! They are the best IEMs for that specific situation I have ever used. No more rail noises.
Well, the ZS4 seem to isolate more than the ZS3. Maybe because the internals are stiffer now without that heat-shrinking bad excuse of a tube.
Not only they isolate way more than the majority, the bass suffers less from transport vibration. It keeps its pace and definition. Kick drums are still there and stuff. This is a rather good option for you drummers out there!
What I like most about the ZS4:
-price
-form factor and comfort
-bass definition
-price
-controlled and "au-point" sub-bass
-"elegant" midrange when using foam tips - the mids have a nice organic feel about them, a good timbric response, although they are indeed a little too low
-rather good instrument separation
-price
-treble definition - these don't hold back on the high key, they sure have quite a formidable range there, and it accounts for a spacious sound. Stage is tall enough but not very wide nor deep. Just good enough. In fact, excellent considering the price. Nothing to complain about.
What I like the least:
-some sibilance. I don't know if it will get tamed with use, burn-in, brain burn-in or whatever. By now it is there sometimes with some sounds. It gets better with foam tips.
-mids when using silicone tips (starlines, Sony hybrids) are not as resolving as with foam tips, losing some of the weight and resolution. They get a little thiner...
-the "y-split's edge of death" (photo below). C'mon KZ! Don't give us a y-split with that sharp edge in that particular position that it is prone to get stuck on tabletops!!!
-the cable is good, and the black is back! (some older KZs from the beginning of times had black cables). But I noticed it only has 3 wires coming from the jack as opposed to other KZ braided cables that have 4. No big deal here, only for 2 things: it is more prone to unwind the twisting - and I can already notice it is untwisting at the jack's strain relief; imagine the "balanced" thing ends up being The Thing for the future - now you have to get another cable instead of just soldering a different jack!
-see, no metal memory termination anymore, because I am a master in the x-acto/scissors/pliers removal technique. The memory wires are awfully uncomfortable and painful and awkward.
-it should be mandatory for KZ to include a set of (good) foam tips in the package (not that crap that usually came with the ATE). This is not the case, but for flagship KZ models they have that useless metal plaque that costs money. Why don't they cut the cost and throw everyone their foam tips?
-glossyness - thank the Flying Spaghetti Monster my fingers are dry, but these glossy shells are fingerprint magnets. The matte ZS3 is still handsome, IMHO.