The M16A4 is the same as an M-16, except that the carrying handle has been removed and replaced with a Picatinny rail that allows the attachment of more accessories.
The M-4 is an upgrade of the old CAR-16. It is a shortened M-16 (carbine) that also has an adjustable stock. It is claimed that it is as accurate as the M-16 in spite of the fact that the barrel is significantly shorter.
The C-8 is the Canadian built version of the M-4, it is slightly different in appearance, but underneath it is basically the same thing.