I do have 1 data point. I have heard that H/K receivers are very conservatively spec'ed. It was either the 8000 or 7000 that was reviewed in S&V, and they said that it was was of the only receivers that they have ever reviewed that could do 100W per channel, *all* channels driven. Most receivers' power falls off as you drive more channels.
Shoot, and there was a thread somewhere talking about a receiver that had problems at 4 ohms... (The manual even recommended to *not* use speakers rated that low, and someone posted that it kept shutting off on them.) Maybe someone will post what the manufacturer was...
I know Denons are stable into 4 ohms, depending ofcourse on your speaker sensitivity, your listening SPL and receiver ventilation. Denon's web site FAQ states that their receivers shouldn't have a problem driving 4 ohm speakers within reasonable volume levels. I would think Onk and HK should also be stable given the above considerations. As long as your speakers are sensitive enough (>86 db/w/m) and your listening volumes are reasonable and as long as your receiver is well ventilated to dissipate the extra heat from driving your 4 ohm speakers, you shouldn't have a problem.
Not a reciever, but I used to have an H/K Signature 2.1 amp. It drove 8 ohm Klipsch loads for several years with out a problem. Then just before I was about to sell it I hooked it up again to test its working status to my new 4 ohm speakers. It was going well at low volumes then I decided to turn it up a little closer to my average listening level, you know hadn't heard it in a while. Boom, there goes the left channel fuse, not exactly sure but more then likely this was just the amp saying I give on driving this load and thus protecting itself.