Looks are the same. Both are flattop carbines. The decision to install the rail adapter system is a unit one, but the M4A1, due to it being used by SOCOM, will have one 99.9% of the time. The M4 has a three-round burst, while the M4A1 is full auto. Like I said earlier, M4A1s are used by SOCOM, while M4s are used by the regular ground-pounders, so they are not as likely to get all of the bells and whistles, unless the unit commander makes the effort. When my division traded its old M2 Bradleys for a newer model, one battalion commander decided that he wasn't going to pay to keep vehicles that he was going to turn in in top shape. He made sure that the Brads could drive to the turn-in yard and spent the rest of the maintenance budget on accessories for his M4s, gear for the snipers, etc., so that unit almost has the equivalent of the SOPMOD package for every fire team. Not a bad decision in my book, but they still only have burst, as opposed to the real SOPMOD M4A1s.
"If ye love wealth better than liberty, the tranquility of servitude than the animated contest of freedom, go from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains sit lightly upon you, and may posterity forget that you were our countrymen!” - Samuel Adams