Personal experience...15 years in the Army...
It depends on the unit. When I was in artillery units, we got M16s. When I was in the 101st (light infantry division), we got M4s. However, I think these days (I left in 2014) everyone on active duty is carrying an M4. M16s are literally muskets compared to the M4 and just super inconvenient.
As for as add-ons, I can't speak for every facet of the Army, but in my units, VERY FEW personal add-ons were authorized. Mostly just additional magazine holders for the stock and such. But you had to pay for that yourself. Scopes were not authorized whatsoever and quite frankly, they weren't necessary at all. If you need a scope with an M16/M4, you need to spend more time on the range. Designated snipers were the only exception and even then, they usually carried an M14 in a combat zone anyway.