you can refer my Xiaomi Piston 2 review as an introduction to this one, because this two are indentical in general. the only difference between they're, is the frequency balance i think. M4 comes with 125-250hz strong bass frequencies amplified on somewhat level. M4 is overall sounds better but as i said, they're pretty much identical with each other.
 
comparing the M4 to M5, with M4 you'll lose some bass & treble compared but, "Tuned for Vocals" title on yourbrainwavz.com is for real this time (surprisingly). so you'll have good seperated, clear vocals with M4. i also detected some 4khz frequency amplification on the device (but on a modest level, not like 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 prices, 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. i sold it but it also reported dead after 3 months. i found a lot of people in the forums who faced the same situation.
 
but i must say, brainwavz-mp4nation is very good about the warranty support. they have a real, painless warranty service but sound quality is more important and this is where M4 falls down. if you wanna buy a good earphone for the price, keep out from this and alike. there is a lot better models out there, both for the piston and M4-M5.
 
btw, i was a little "mp4nation beginner" at the time of purchase and i bought it from a reseller and i just can't believe i wiped 45 washingtons for this.
 
visit my table for further comparisons and informations

i know you already saw all the threads, reviews, comments speaking about the sweetness and cheapness of this earphone (eartrash actually). i also know that there's a common talk about it's fakes, which makes you feel like if you can get the genuine one, it's going to be the perfect earphone of the stars... there's also a talk about it's "beryllium" that you don't even know what it means. i know all of that makes you wonder this item more and more...
 
i know you want all of this turn out to be true... you want a cheap earphone that beats the rest of them. but for this particular earphone, all i'm going to say is: just don't buy this. don't spend your bucks for nothing... the people you see in the forum threads (and review pages), acting like there's no better alternatives and it's price is actually $1 or something. for them, the world of budget earphones is collapsed and only piston 2's are left.

actually, if you buy that piston they talk about, you'll be left with a very small acoustic (acoustic = soundstage, soundfield, positioning, separations etc.) which means unrealistic sound reproduction and below average sound quality. even if you manage to live with this kind of bass and treble performance, you're inclined to feel something missing between the bass and treble, the most important part, mids are suffocated.

this earphone is actually nothing new, pretty much identical to brainwavz m4 (except some frequency balance differences) sounds like cheap earphones from big companies (like sennheiser cx or creative ep series) same compact design, same unrealistic acoustic reproduction, same below average performance which excels in nothing.
 
come on, offer me $300 per month just to use this "thing". i'll not accept. just for $300 every month i'm not going to sacrifice my entertainment that much. if you up your hand to $500 maybe we can get to somewhere but that's just a maybe... you see the problem ?
 
bought from penonaudio.
 
if you think piston 2 is good just because of it's price tag, please take a look at this list first. they're my favorites and they're the real thing. if current prices are different than this list, i say wait for a discount or just try to spend a little more:

audio-technica cks77, $16 on aliexpress.
brainwavz beta, $9 on amazon.
koss plug, $10 on amazon.
moxpad x6, $15 on aliexpress.
 
notes: beta is the best of brainwavz (not treble wise), do not let prices cloud your judgement. cks77 sounds exactly the same with brainwavz beta. koss plug needs extreme equalizer reductions to sound normal. moxpad is an alternative but only if plug or beta-cks77 is not the choice for some reason.
 
visit my table for further comparisons and informations