According to Denon's Home Theater FAQ section:
http://www.usa.denon.com/support/faqs_ht.asp#Q9
"9) CAN I USE 4 OHM LOUDSPEAKERS WITH MY DENON RECEIVER OR POWER AMPLIFIER?
Yes, you certainly can.
To understand this a bit better, first realize that all amplifiers are designed to deliver a signal into an electrical "load" or resistance presented by the loudspeaker. We measure resistance in units called "ohms" (after the German physicist Georg Simon Ohm, 1787–1854).
Conventional wisdom makes an 8 ohm loudspeaker load the most acceptable because it "protects" the amplifier from delivering too much current. A 4 ohm loudspeaker can encourage a marginally designed amplifier to deliver more current than it comfortably can.
However, you should remember that a loudspeaker’s impedance rating is a nominal or average one: A speaker rated at 8 ohms may actually vary from 5 (sometimes even less) to 20 ohms or higher, depending on the frequency at which you measure the impedance. (Don’t worry about this too much -- good speaker engineers are well aware of these variations and take them into consideration when designing products.)
In general, you’ll find that Denon products are designed to function with a wide variety of loudspeakers and have power supplies and output circuitry more than able to meet the current demands of low impedance loads.
In the rare event that very low impedances tax the amplifier, quick acting circuitry will protect it from damage. If unusual operating conditions trigger this circuitry, the word "PROTECTION" will appear on the unit’s front panel. If this happens, simply turn the unit off, wait a moment or two, and turn the unit back on again. The protection circuitry will automatically reset. If it re-engages, check your system for possible malfunctions."
It seems that Denon claim they can drive 4 ohms load but it could trip under "rare" conditions. Add a 2 channel amp and you will have no worry.
