world's greatest " progression " puzzle

This problem was sent to us by the famous Birdman of Altoona. I just hope that the canary above is cage-broken. Below we have a series of three numbers, 4 , 1, and 2.
He want to know the next logical number in the sequence! the answer is one of the following numbers, 6, ,7 ,8 or 10.
also explan your logic !!!!!!