Hello and welcome,
M4 mags are identical to M16 mags. Look around a little more and you'll find many sites refer mags as "M4/M16 mags". Also, if you buy a gun, chances are it will come with a hi-cap magazine (if it's new - as in from a company or shop). Hi-caps are great for their capacity (no shit) but in terms of realism, they're annoying to some other players (go figure) because the rattle all the BBs make when someone runs. It's detrimental to both other players and yourself. I'd suggest mid-cap magazines because of their pretty high capacities (normally from like 70-130 rounds/mag) and the fact that they do not rattle. A hi-cap magazine requires its user to wind a little wheel at the bottom of the mag to bring the BBs up to feed them into the magwell (basically the gun). Mid-caps and low-caps do not.
Mid-caps have more capacity than low-caps in terms of BB storage, but that's about it when it comes to comparing low and mid-caps. Low caps I think are storage replicas for real magazines (ie. 50 rounds for P90 would be a low-cap). Some US sites have some excellent deals for mid-caps. I got a pack of 8 MAG mid-cap M4/M16 mags from Airsoft GI for something like 60$. That's the price for 3 mags purchased individually. Take your time to shop around and make sure the site you want to buy from will ship to Canada (and whether you're willing to pay the shipping costs and duty fees).
Hope it helped... good luck!
Last edited by .JET; August 24th, 2009 at 23:05..