While the Denon manual gives power output at 4 Ohms, the back panel states ranges for the speakers at 6-16 Ohms. It might not have a problem with those speakers, but I tend to agree with lsiberian as it might be risky - especially at high volumes.
Were you planning on hooking them up to the assignable amp outputs, or to the "B" speaker terminals? If hooking to the "B" terminals was the plan, then I would say definitely do not do that as 4 Ohms would be too low for that.