Go to HK's website and see FAQ's, they "imply" that HK is stable @ 4 ohm. I may run 4 ohm on some of the speakers on my 525 I just got, ( have not yet tried out the receiver) so we will see.
I run 4ohm mains on my 325 with no prob. It get's fairly hot but it hasn't shut off yet, plus I only have about 2" clearance from the top 1" to the right but open to the back and left.
On my parents 525, I setup the receiver with an axiom epic 80 speaker system. The fronts are 4 ohms, the center and 2 rears are 6 ohm. I was able to turn the receiver to a high volume with no problem. I think it will handle 4 ohm loads fine.