Many 4 chnl amps on the market only need one set of RCA cables connected to the "AMP-1" inputs to run all 4 chnls. Some will pass through the signal automatically, while some just need to change the position of a switch. If your amp will not do this, then all you need is a pair of Y cables to split the single left and right RCA's into two.

This will still leave you an unused output for a tower or sub addition in the future.