Originally Posted by Steve_Si
That first 4 channel amp will run all four speakers just fine. It will be capable of sending 50 watts RMS to the front speakers and 100 watts RMS to the rear speakers, due to the lower impedance of the rear speakers.
It is rated for 2 ohm stereo, which is perfectly fine for running 4 speakers, even if one pair is 2 ohm and one pair is 4 ohm.
Lots of people use a 4 channel amp to run two 4 ohm speakers at the front, and then bridge the two remaining channels in to a 4 ohm mono sub. Whilst technically different, your setup results in the same power levels being used.