A couple of weeks ago I e-mailed HK and ask them about using their AVR receivers to drive 4 ohm speakers and here is the reply from their specialist:
...The AVR models are rated at 8 Ohms. The higher power units can be connected to 6 Ohm speaker, but remember the speaker impedance can drop during peak transients. If you start off with a 4 Ohm load you can drop to below 3 ohms at peak. This will shut down the receiver into protection mode should this occur. They will work but you can run the risk of it shutting down on you during playback if output is at a moderate level...
I am already going to get a Rotel for my 4 ohm speakers. Just post this as a FYI.