John,
I can't seem to locate any 4 ohm selection in the manual or as I flip though the setup options on screen. I checked the speaker and audio setup options, and couldn't find a thing. Not sure what section its under on the 797 menu (perhaps chess can let me know where it most likely would be).
On another interesting note, on the last page of the manual it Onkyo rates 'dynamic' power at 250W down to 3 ohms on this sucker. Interestingly enough they rate the continuous average power with all channels driven at 100W RMS for 6 channels driven....surprising since power consumption is rated at 440W. The mystery deepens..........
