Job number one would be take it to a friend (or to site) and try out whatever mags they're using to make sure whatever brand you're intending to buy will fit and feed. I'd say MAG are good, but my M4 just didn't like them; it's a G&P but it works best with CA mags.
I've seen two threads recently where people have just bought 'M4 mags' only to find out that the ones they bought didn't feed (or in one case even fit) in their gun. My response to one guy (and it would be the same to you) was:
In my magazine box upstairs I have 6 different brands of M4 midcaps. This is because over the years I've had about 5 different M4s and they were all just picky about what they would feed from (in some cases some mags would not even fit into the magwell). Unfortunately not all M4 mags are the same, so while one gun may take one brand of mag it won't necessarily take another brand, even if they look the same. My G&P likes CA mags for example, but won't even fit Dboys and doesn't like to feed from A&K or (strangely) G&P.
It's not just M4s either. I just bought an LCT AK which came with an LCT mag that feeds fine. I was then happy to find out that my TM mags from my TM AK also fit and feed fine, but confused to then find that my SRC mags, which fit and feed fine in my TM, will fit but not feed in my LCT.
So it's worth checking (if you can) before buying!