Welcome to Axiom. Amplifiers, including those in receivers, have to be rated in accordance with the FTC amplifier regulations. Among other requirements, at the rated impedance at least two channels have to be simultaneously driven for at least 5 minutes at the full rated power. This is a severe requirement which can be measured in the lab, but doesn't occur in home use. Receivers can meet it at 8 ohms but not at 4 ohms. There are no "4 ohm" receivers, such as you mention.
This should be of no concern in driving your Axioms, including those rated at 4 ohms impedance. Any quality modern receiver, such as those which you're considering, carrying the usual official FTC 8 ohm power rating can drive them to all safe(to hearing)listening levels.
So power isn't a problem, and your decision should be based on the features in the various units. In particular, of those you mention I'd suggest that you consider the Denon 4200 with the the highest grade Audyssey MultEQ XT32 setup and room correction processing(Onkyo no longer uses it and has replaced it with a cheaper and less effective in-house system). A good buy is at
Accessories4less for the Denon 4200 factory refurb, which should be, if anything, even more reliable than a brand new unit.
