The real PTS Pmags (not the green label box sets or beta project pmags) work very well in the KA M4's. Another option is the Socom Gear magazines, be it the Lancer L5's or Troy Battle mags. I have had good luck with these working well in the KA M4's.
If I remember correctly mag brand magazines also work but I may be mistaken. All of these magazines can be bought at airsoftparts.ca.
Durate et vosmet rebus servate secundis.