You can buy a subwoofer with a built-in amp. You can buy a five-channel amplifier instead of a four-channel amplifier. You can buy an amplifier where the rear channels can work in what's called "Tri-mode" where the rear channels power the rear speakers and the subwoofer at the same time.

I agree with Doug on the importance of a subwoofer. I would almost even consider adding a subwoofer BEFORE replacing any existing speakers.