M4s do not use the flip-up covers like the M2s and M3s & related variants - the M4 comes with an Aimpoint factory supplied bikini cover....
The flip-ups were getting broken coming out of vehicles and banging through doorways, etc., so the military went back to old school (read: cheaper to issue as G.I.) rubber bikini covers...like the ACOGs use.
If you "have to have" them, send Aimpoint Customer Service and Sales an e-mail - they can tell you whether the other covers are compatible....
I like the flip-ups better [use them {GG&G} on the EOTech too] - the rubber cover on my ACOG smarms the lens up with rubber residue and I have to clean it more often - minor PITA but still....