People do it all the time. Most of the receivers can handle it. If not, they'll just shut down temporarily. Usually no harm done. I'm not running a 4 ohm load, but I am running 2 sets of 8 ohm for L and R and C channels in parallel with a Pioneer Elite model.

If the receivers couldn't run the 4 and 8 ohm loads at the same time, then they couldn't run the M80s and QS8 surrounds at the same time.

https://docs.google.com/spreadsheet/ccc?...amp;usp=sharing