Here is a quote from an audioholics review on the subject. Basically you can do it if you don't play too loud I guess, but it does sound a bit doggy to me.
In reply to:
Driving 4 ohm loads is certainly not the RX-V2500’s forte, but it did a commendable job considering. It was able to deliver 200wpc with one channel driven and about 110wpc with 2 channels driven unclipped. Initially I cranked more power out under these test conditions from 20Hz to about 15kHz but then found major slew induced distortion at higher frequencies followed by a rapid shut down of the receiver. This is very good news actually since the amplifier automatically shuts down once bandwidth uniformity has been compromised for being driven too hard and is a good preventative measure for protecting both the amplifiers and the loudspeakers. This shouldn’t be a real world problem because at high frequencies very little power is being delivered to the speaker system since music is mostly harmonic in nature above 13kHz or so. The real power demand is in the bass and in most home theater systems a dedicated subwoofer or two is used to alleviate this problem.
Considering this, I backed off on the Yammie until I had a pristine unclipped waveform at 20kHz to come up with the measurements published herein. It should also be noted that anytime I drove 4 ohm loads at full power, the fan came on shortly after. I caution those using 4 ohm speakers to make sure you have plenty of ventilation and airflow.
Recommendations
The Yamaha RX-V2500 has respectable amplifier performance for a receiver in its price class and should serve most home theater environments quite well. I advise against using low efficient 4 ohm speakers in environments where you like to play your music loudly and your room dimensions exceed 2000 ft^3. For best results, my advice is to use reasonably efficient (88dB or higher) 6-8 ohm speakers (4 ohm may be used if their efficiency is 90dB or greater and you provide adequate ventilation) and apply bass management to all channels and route the power hungry bass information to a dedicated subwoofer or two. If you find you need more power in the future, simply preamp out to a dedicated amp and you have yourself a great processor with all the latest surround formats on board.
Tuff call.
Link to the article 
