No speaker manufacturer, including Denon will recommend using a 4 ohm speaker for liability reasons.
I think that the ratings on receivers are loosely based on 2 channels driven. The problem with Yamaha is that, for the lower end stuff, they are hit and miss. This was covered in a recent post here when Alan loft pointed out that one of the 66x receivers delivered considerably less than the implied 100w. In contrast, when Audioholics bench tested (and their bench tests are comprehensive) the 559 it delivered more than the rated 100wpc 2 channels driven on a contiuous basis.
The M60 will go plenty loud. They may not fill up a factory with sound like the 80s, but they will go plenty loud in most average and larger rooms. When I a/b compared the 60 and 80 in a 20x12x8 room neither was playing at near even half power to deliver 85db at a listening position 10-12' from the speakers.
There are room calculators out there that will tell you how much power you will use to reach a given volume at a given distance. In my room, almost the same size as listed above, 65w gives me 12-15db of headroom at average lisening levels of 85db. What that means is that the speaker draw will be 65w ONLY for the 95-100db peaks, and at 85db I am using considerably less.
