Sounds like you have 1 front/rear rca output and 1 sub output.

Most 4 channel amps only require a single rca input to run all 4 channels on the amp. There should be a switch that says 2 ch/4 ch. In your case, you would choose 2 ch because you only have 1 RCA available. If the amp did not have this feature, then you would need to get a pair of Y splitter cables that will give you 4 outs instead of 2.

The sub preout is simple, so I won't go into that....