Well belive it or not, in normal listening you only use 5-10 watts max. So your reciever will stay well within its posted distortion ratings. Now what you will want to do is do some decent listening for a maybe an hour, or just watch a good loud movie like Days of Thunder Or Saving Private Ryan, and then actually feel your amp. If its its relativly warm your OK, if its pretty hot, then you may have a problem.
For all you know the switch on the back may parrallel the output devices thus making them "see" an 8 ohm load. Thus it will behave as normal. But either way most new recievers can handle 4 ohm loads, because as I stated you usually only use a small portion of the ouput power capabilities, so its able to disapate the extra heat.
