How to run 4 channel amp?

I have a Kenwood 9103D amp right now. 900W RMS mono for my subwoofers. im going to replace the speakers soon but how am i going to get both amps workin
How to run 4 channel amp?
first get ya mind outta Q/A then go on with yore business, how does that grab ya%26#039;%26#039;