DD5.1 requires 5 discrete channels, plus LFE output for a sub. If you have a 4 channel power amp, I would suggest you use three of those channels for your front L&R speakers and center channel. Use the receiver's L&R rear outputs direct to the L&R rear speakers. Most of the sound, and power consumption, are for/from the center channel and front L&R speakers. Powering the 2 discrete rear channels directly off of the receiver should be fine.
Don't forget to re-run the setup program after you insert the 4-channel amplifier into the mix. The setup program should compensate for the added power being used for the front and center speakers.
Don't forget to re-run the setup program after you insert the 4-channel amplifier into the mix. The setup program should compensate for the added power being used for the front and center speakers.