Most home consumer product is designed to drive an 8 ohm load. A higher load (like 16 ohm) is safe because the amp is lowering it's output based on the higher resistance. However, running lower than 8 ohm will cause the amp to attempt to put out more power than it was designed to do resulting in more heat which will damage the unit. Some home audio speakers are 6 ohm and I imagine that the receiver will drive that load but you will probably shorten the life of the amp. I would avoid 4 ohm at all costs.
Also, wrong forum.