I went through a couple Onkyo's. With the 4ohm load a fan is highly recommended. Even Integra gets really warm with 4ohm. Haven't seen anything about any circuit melts or other problems they had with older models in recent units.

The AVR is more likely to take some clipping and go back to normal.
The speakers are in danger without a lot of power. Though not playing reference probably not a worry just keep an eye on the unit. They can get hot really fast.