M16 Vs M4, someone clear this up for me?

Not too familiar with these weapon systems, or any weapons in use by the US to be honest... why do you have 2 rifles in use? What are the advantages/disadvantages of using the M16 instead of the M4 and vice versa. I know that the British Army use 2 different rifles, they are the L85A2 SA80 and the L22A1 Carbine, the carbine issued to tank/armoured vehicle crewman and the L85 in standard use. I know this isn't the case for the US though so can someone clear this up for me please? Thanks in advance!