Wiring 4-channel amp to run front comp's and sub

[wapam]

Don't have an amp yet but thinking about Kenwood eXcelon XR-4S which has RCA inputs. I get the output, channel 1/2 to the comp's and channel 3/4 bridged to the sub but I can't get the inputs straight in my head. I also want to run the factory rear speakers off the factory H/U. 2011 WRX no nav.Thank you for any advice.

[EJ257]

Ch 1/2 for the speakers
Ch 3/4 bridged for the subwoofer

Which components? Which sub?

[T0rque]

you can get an adapter to run pre-outs of an oem deck

[wapam]

I understand the outputs, 1/2 for the speakers and 3/4 bridged for the sub (changed my original post to be more clear) and I know I'll have to use a line output converter, I am interested in how to wire the amp's inputs.