Mag brand mags are some of the best on the market. Not 100% sure if they are compatable but they should be with 90% of guns. I find cyma stuff kind of works only with cyma stuff alot of the time. My mp5k wouldnt hold mag brand mags and i was stuck buying cyma mags. I know their 416 (baisically an m4) only likes cyma mags so id guess that their m4 mags dont play well with others.
Edit. A quick google search shows that the mag brand mags work fine with the gandp m4's
Lotta mags right here