I've got a 4308 too, so I dug out my manual. Page 90 of the troubleshooting section suggests a few possible causes:
1. The speaker cables up on the receiver's rear panel have some cable strands touching each other, causing a short. Not possible with banana plugs, but if you've got bare wires, it's worth a thorough check.
2. The receiver's too hot, so it has activated its protection circuit. Not likely unless you've got it in a cupboard with the doors shut and a pile of kit sitting on top of it, and it doesn't sound like you have, because you'd have mentioned it!
3. Speakers with a difficult impedance are being used - again, not likely. It's a powerful receiver, and most mainstream speakers won't cause it any problem.
4. It's knackered!
So if the speaker wires aren't an issue, I'd suggest you get on the phone to your dealer straight away.