Mil-Spec M4A1, as issued today, includes the KAC RAS, MaTech rear sight, CompM4/S or ACOG, a sling and foregrip. The only addition that isn't always issued would be a light, and maybe the foregrip/rail covers if one doesn't get the full kit.
That's fairly basic, all's said and done, and it seems to be the best definition of simplicity; everything you need, and nothing you don't. An optic is a necessity for shooting in the dark, and the rail is the best way of attaching a light. It can also multi-task, allowing your preferred sling point and a place to mount a forward grip, if desired.
And as others said, a weaponlight is ALWAYS necessary. Half your life is going to be dark, and if you do not ID your target, you might end up hesitating on a bad guy or shooting someone innocent. Either one can and will cost you your life, either in prison or in the ground. Flash the target to get good ID and blind them, then engage at your discretion. It ain't rocket surgery.