Not at all. The Denon doesn't care what the resistance (I know there's a better word for it, but I can't remember right now) is, it just pushes what is called for. In practical terms, it means that the Denon will run hotter with a 4 ohm load than an 8 ohm, but that receiver will handle this just fine. Other brands have been reported not to.

I am the Doctor, and THIS... is my SPOON!