Onkyo's run very hot. It is completely normal. Driving 4-ohm speakers might make it just a tad bit hotter, but your M80's are not the reason that it's hot to begin with. See numerous AVS threads. It's not a problem unless you have it in an enclosed cabinet with no air circulation.
As JohnK says, it is usually recommended to just leave the impediance switches in the '6+ ohm' setting unless you start to experience thermal shutdowns. Though with the 876 I just returned, I could tell any difference in terms of sound volume or quality when running my M80's in the 4 or 6+ ohm setting.
As a test, just leave it on and don't play anything - it'll likely remain just as hot as it does when you're using it.
Enjoy the fabulous new setup!
