Is this a 4:2 Multiplexor?
With 4 different combination of S1 and S2 (1bit selectors), you select one of the four inputs and get that selected input on the output. So 4 tells you that you can get four different values at the output, and 1 tells you that you can have just one at a time on the output. I hope its now clearer whats happening here.

Your picture is 4:1 multiplexor, and if I'm right 4:2 multiplexor looks like this



Used from The Multiplexer