The Denon won't have any problems with the M80's 4-ohm load, provided you are not trying to fill a huge "great room" with high-volume sound or use plus 100-dB SPL playback levels all the time.
Run within their output limits, all Denon AV receivers have been very stable with the M80s. However, understand that 4-ohm loads draw a lot more current through the output stage, so your Denon will run hotter than with an 8-ohm speaker. Make sure the Denon is well ventilated--4 or 5 inches of air space above it--and you should be fine in normal sized 2,000 to 3,000 cu. ft rooms.
My H/K's auxiliary cooling fan kicks in and roars like a banshee when I run my M80s really loud (it doesn't have enough ventilation above it--only an inch) but it has never shut down.
Regards,
