Cons: Soundstage is too small (especially in height), Mids are muffled, Deep bass lacks body & small sounding, Treble is not well seperated.
refer to my Xiaomi Pistons review for my detailed thoughts about this one, because this two are indentical in general. the only difference between them is the frequency balance. M4 comes with some midbass frequencies amplified a little bit which helps the naturality of the bass sounds usually, but of course that is also depends on the songs played. so if you feel distortions or overly strong bass i can suggest lowering the 125-250-500hz frequencies a little bit.
i also detected some 4khz frequency amplification (but on a modest level unlike R1's 2khz situation). 2-3 desibels of decrement on this frequency will help it's naturality. in general, i personally favor M4's sound to piston but even with the discounted mp4nation price, i cannot recommend it over piston due to build quality issues. mine was gone from the first day. mp4nation sent me a microphoned version after that. it was working perfectly but i found the casing was too fragile both on the two.
build quality does not really shines on this model. if you wanna buy a good earphone for the price, refer to my piston review again for my earphone recommendations in this price range. there is a lot better models out there both from the piston and M4...
note: if you want to upgrade from M4-Piston to a better model and want to keep the soundstage & bass acoustic nearly the same, i mean if you don't share the same thoughts with me on this, i have a good suggestion for you, try Meelectronics M-Duo. it has crystal clear highs & good quality bass (but midbass freqs comes amplified even more than M4). it is definitely an upgrade for those.