I don't have the m50 (I've heard them though) but do have the Alessandro MS1i (step up from the SR80i) & FA-003..
VS the m50 : As NewVegasBaby said above, the 004 trumps the m50 in every category except for bass QUANTITY (which isn't a bad thing considering the m50 can have a overly-dominating bass presence). The m50 also needs an amp for it to really shine.. I think it's adequate unamped but nothing more.
VS the MS1i : the 004 has a much wider soundstage (so it's better suited for electronic music, etc.) and matches it in detail. The MS1i has the typical Grado sound sig (mid forward, aggressive, small soundstage, average-below average bass quantity, suited well for rock) but smoothed over a bit. Instrument separation is on par if not better than the MS1i.. bass quality & quantity is better on the 004.
VS the 003 : The 003 is a definite step up in all areas, timbre, accuracy, detail, transparency.. but the 003 also aims for a different sound signature where neutrality is paramount. The 004 is quite a neutral can too, just with a slightly spiced up mid bass. It isn't really fair to compare the 003 to the 004 cause the 003 costs almost three times as much. Comparing the bass, the 004 certainly has a larger quantity bass but the 003 takes the cake when it comes to bass texture, detail, and layering (as it should). The 004 is no slouch though as you get very very good quality bass from the 004. For it's price, it's unbeatable. FA (aptly) markets the 004 as the headphone for an "audiophile-on-a-budget." I owned the 004 before the 003 and as I said above, I consider them to be very complementary cans if you enjoy the FA sound.
So.. the 004 has more bass QUANTITY than the 003 but less than the m50. I'm a big big fan of the 003, I love the quality & quantity of bass in the 003.. but the 004 is very satisfying bass-wise if you're looking for a fun alternative & want more bass than the 003 offers, but don't want bass that's bloated and dominates the sound signature.. this is coming from a hip hop/downtempo/electronica listener who likes his bass.
I mentioned earlier that I added a P-S adapter to attenuate the 004's mid bass hump a bit and it worked out very very well.. I'm not sure if the adapter did something or if it was just a matter of burning it in more, but I used last night w/o/ the adapter and it has improved quite a bit in terms of becoming balanced. The mid bass hump tightened CONSIDERABLY.. it still extends deep and has a nice thick bass presence, but the entire bass response is much better behaved. The mids & highs are now more prominent, have fantastic instrument separation and detail.. it sounds strikingly similar to the 003 now, albeit with a slightly more "fun" sound sig and more bass quantity. For the first hundred hours or so, they lean on the warmer side of neutral, with thick bass (which can become uncontrolled at time depending on the source and quality of files). As a result of the warmer sound, the mids & highs get pushed back slightly. The thing is, the highs & mids have excellent detail & quality so I knew the potential for them to come out was there once the mid bass recessed a bit.. and sure enough after 150+ hours.. they did and now the phone has really bloomed! The bass is still plenty deep and has great presence, but it's become much tighter and let's those wonderful Fischer Audio mids & treble breathe more.. you'll really appreciate the presentation of these phones once the mid bass gets in line after significant burn-in.
I've got about 170 hours on these phones now so perhaps they just require quite a bit of burn in. I listen to the 004 on a nightly basis so I'm quite familiar and adjusted to it's sound.. it underwent something drastic these last few days & I'm very very very much enjoying it. I was actually considering jumping on the 011 ASAP just to see what a refined 004 would sound like as LFF said it essentially improves on it on all fronts.. I'm happy to say I'm going to hold off for a little while now since the 004 has reached what I would think it's potential now. Also, it's very easily driven unamped so it's an excellent portable phone, opposed to the 001 which require an amp (portable or otherwise) to sound best.
So, I definitely recommend getting the 004 over those other cans and let them burn in EXTENSIVELY.. I think you'll be quite happy. The 004 sound GREAT unamped (unlike the m50) & has a sound sig thats suited to a wider range of genres than the SR80i. It has quite a good soundstage for being a closed can too.