4 Channel amp hookup question.

I have a pioneer head unit with a front speaker rca output, and a subwoofer output. I am purchasing a 4 channel amp. My questions are this: number one does the front rca output on my deck control all 4 front speakers, or just the front two speakers? My original plan was to bridge the amp to run the subwoofer on the rear channel and then run my front and rear speakers off the front channels, but i have heard that if i do this, i will lose the fader control on my front speakers. Is this the case? Am i better off running the 4 channel amp to my front four speakers and purchasing another amp to drive my subwoofer? If there is a way to wire it so i do not have to buy another amp, i am all ears, but i get the feeeling that i would be better off with the second amp. If anyone has tried this could you help me determine which is the best way to go! thanks! 

What model is the Pioneer HU.

I am more familiar with HU's having only Front and Rear RCA's or only Rear and Sub RCA's.  It is possible (Likely?) that your HU has a pair of front and a pair of rear/sub RCA's, but I would need the model number to check.

Assuming you run the front four speakers off the front channel and the sub off the rears, you will lose the fader control.  You may also run into problems matching the impedance to what the amp is looking for.

Best bet is likely a separate amp for the sub.

But I'm making a lot of ASSumptions without knowing the HU model.