Make sure you have it properly ventilated and hopefully your room is not very large so you are not driving it at +18 db to achieve reference levels
I think you should be just fine. Let us know of your experiences in either case.
In the FAQ section in Denon USA site they explicitly state that their receivers can run 4 ohm speakers. So running them IMO shouldn't create any warranty issues in case you fry your receiver.
you should be fine. I drove martin logan's with mine for a while and they are nominal 4 ohms and dip very close to 1 ohm. pay attention to heat though.