Cons: Soundstage is too small, Mids are muffled, Deep bass lacks body, Treble is not seperated enough, Overall detailless sound
refer to my Xiaomi Pistons review for my detailed thoughts about this one, 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 to lower the 125hz frequency 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 ($30), 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-M5...