Can I mix ohms in a surround system?

Front and center 4 ohms, rears 8 ohms? Can I do that?

If so, does the equipment detect that by itself, or do I have to set switches or somehow tell the amp what the config is? Any downside or reason not to?

The amp is the Emotiva XPR-5 which handles both 4 and 8 but I'm not sure if you can mix them as above.

Thanks - rev
Rev, you certainly can. My rears are 8ohm in-ceiling speakers and my fronts are 6 ohm. So specifically, you can have different impedance speakers on the same amp.

In my opinion, more of an issue is the timbre match of the speakers as opposed to the Ohms. If the speakers are from the same manufacturer, I wouldn't think twice about it.
Cool. What about setup or configuring - do I have to switch anything or it just doesn't care?