I don't think there is a functional/compatibility difference between m4 and M16 magazines, in airsoft, from what I can tell it's purely cosmetic.
I ascertain m4/m16 magazines are available in capacities of:20,30,40,50,79,80,82,85,120,130,140,170,300,350,450,850,2500 and several others.
I have no idea where the transition point between low and mid cap falls on the list above, I don't know whether low cap are just "anything wot is realistic".
Personally I think I'll be looking to run with four 120rds and six 30rds mags.
So, what makes a good airsoft M4 Magazine? This question absolutely goes to cosmetics too