#[member='CeDrIc']
i feel that m2 provides a better soundstage and a more tighter bass than the s4.i am a noob in all this so dont really go on my opinion.Saying that,have used both of them and i personally rate m2 higher than the s4.i always find a little less detail in the s4 and sometimes i feel the sound is a bit muddy.
if you are not a basshead then, i will recommend you to go for m3 which is a top quality iem and provides wonderful balanced sound.
i do give you more recommendations if you can be a bit more precise over your budget.