[L473ncy]

M4 is basically just a shortened version of the M16. There are a lot of things to wrap your head around.

The difference between M16A1 vs A2 is basically the hand guard. Other versions (A4) have a flat top for optics.

You can have a lot of different options for the AR-15. From front wired PEQ boxes, to MOD stocks, to iron sights. It's a lot of stuff to understand.

What you should really do is break it down and figure out what you want.

Body: Flat top or iron sight/carry handle
Front end: Rails or regular handguard
Ironsight: What kind do you want? There are a lot of different iron sights available from a lot of companies.
Barrel length: 10.5", 14.5", 20"?
Stock: Crane, MOD, full?
Accessories: PEQ, flashlights, tracer unit, suppressor etc.