FWIW I was also a little leary of running 4 ohm speakers with a 6-8 ohm rated reciever (Denon 3805). A Denon rep told me the 3805 was 3.2 ohm stable so I figured I'd give it a shot. No problems at all,....I pushed it hard for 6 -7 hours. At least I thought it was a good test anyway, averaged 96db measured from 18' away.
So I should think your Yamaha would do OK,.... I'd try it, and if you have some clipping or other problems then look at an external amp, if there are no problems then just enjoy it ;-)