Daniel, you don't know until you try. The fact that a receiver is officially rated for only 8 ohm speakers means that the rather severe FTC and UL tests required for a 4 ohm rating didn't occur, but doesn't necessarily mean that in real-world use one 4 ohm load couldn't be driven without shutting everything down. You also don't need to worry about underpowering because of the parallel operation; this actually takes less power. In any case, the indication of inadequate power is noticeable distortion on brief peaks, not more subtle effects on frequency response,etc.
