Mix match 4/8 Ohm speakers on one amp?

I'm looking to buy a pair of 4 Ohm front speakers and I already have a center and two rears at 8 Ohms. Can I mix the 4 and 8 Ohm speakers on one five-channel amp that is rated for both 4 and 8 Ohms by the manufactuer?
Yes. Each channel is independent of the other, and each amp will react to its own speaker load. I have the same situation myself, using an Acurus 5 channel amp. I do not recommend this set-up, however, as it causes uneven volume across the soundfield. I am trying to dump the speakers I have and move to a matched set.
This should not create a problem so long as:

1) you can set the volume levels for the fronts independent of the center and rears via your processor.

2) your amp is PLENTY sturdy enough to do the job that you want it to. In other words, the amp NEVER sounds strained or distorted at any level of listening that you are likely to make use of.

3) the speakers are not highly reactive or have severe impedance dips

Good luck and hope this helps... Sean