There are quite a few excellent options to go with. I think it's best to look at each one and pick the one you like best, since most M4 variants (and high end AEG's in general) will end up shooting at about the same FPS.
I have also good things about KWA's, G&G's, and I happen to like my CA M15A4 a lot. I would start with those brands and see what you can find that suits you. Chances are that one of those brands has a suitable M4 of a particular type that you're looking for.
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