I would say the most important par for any gun, not just the M4 would be to stick with the real steel, i.e. adapt everything from the trigger system (which on AR15 types is laughably simple) the barrel assembly.
On the front end part I would use a standard delta ring and handguard of a M4A1 to keep it as simple as possible, if you respect the normal dimensions (I'd take Madbull as a reference or a simply a real steel so I can put my stuff on it) of a the gun then people will be able to switch parts.
Also including a hop up system like WA. As painful as it may be to tweak it I like to have as much compatibility as can be.
Tous les jours nous nous endormons plus instruits mais pas forcément moins bêtes...
I can do a CTRL+ATL+DEL on your gun !
Gun doc in Montreal on AEG, GBB, GBBR and Bolt rifle (gaz and spring)
Heavy custom builder (including painting)