Many four channel amplifiers would work out fine. 2 ohm configs allow the amp to put out more power (thus the uneven rms ratings would be compensated easily), and most 4 channels will play 2 ohm on each channel. It doesn't matter if you havae two 2 ohms on the front and two 4 ohms on the rear, it's fine. An Alpine MRV-F340 would be a good amp. Though, honestly, it's a waste to amplify rear speakers. Put your money into the fronts, you'll be much happier.
