The M4 is a fine choice for a first primary rifle. As others have said, there are lots of modification parts available for that particular style of rifle. As stated above, ICS has a lot of proprietary parts, mainly because it's based around an in-house developed split gearbox (splits into an upper and lower). The design is convenient for swapping out piston/spring components quickly (it can be done in about 5-10 minutes), but it also means that replacing the gearbox with something better is pretty much a no-go as all other gearboxes are single piece drop-ins.
Regarding your question about an M16 vs. M4, the M16 is a full-stock, full barrel length rifle originally introduced during the Vietnam war to replace the M14. The M4 is a variant of the M16 originally designed for commandos, and fitted with a crane stock and shorter barrel assembly. The main advantage of an M4 over an M16 is its variable length stock, which allows it to be used for CQC or long-range engagements. The adjustable stock also allows for a person of any size to use the rifle comfortably.
Hope that helps.
VFC SCAR-L - VFC SCAR SSR - KJW Hi-Capa 5.1 - CAW M79
The Three Sisters - WE G39 E/K/C
Originally Posted by kalnaren
Stalker stays where he is.
His BB's fly across the country to hit their target.