Ahhh yes, the old ratings game! Never mind what they say the power out is, what matters is the power in,or how much it draws. The power out ca be expressed in sooo many ways that an honest 10 watts RMS can be 17 watts peak or 34 watts peak to peak, and depending on how much you ram into the input it can be even more watts.
What you want is a power amp that accepts speaker level input to the amp for the easiest wiring install. That way we can just input the speaker wires into the amp and add new wires from the amp to the speakers. Otherwise we will have to butcher up the wiring to get the power amp installed at line level.
Doing the install via speaker wires level will only amplify one pair of speakers though, to do all 4 will either require 2 amps or a X4 amp and that will effectively disable the fader function. Just how loud do you want the system to be???