The most common standard for speakers is "8 ohm impedence", so most amps are optimized for that standard. If you connect a higher impedence speaker or headphones, the speaker/phones will draw less current for any given volume control setting... and most receivers don't have a problem with that.
If you connect a lower impedence speaker to the receiver, it will draw relatively more current for any given volume control setting. Most high end products also have no problem with that, but a few will detect either too much current or too much heat being generated (both only happen when playing real loud) and the receiver's protection circuits will kick in and shut it down.
There are some funny legal reasons (some UL regulation) which prevents manufacturers from specifying 4 ohm behaviour even if they support it just fine, so you have to rely a bit on word of mouth and anecdotal evidence.
Denon, HK and NAD products all seem to be fine (any model). Most of the high end products from other vendors also seem to be fine but unless we know of someone successfully running them with 4 ohm speakers we tend to stay away to avoid the hassle.
You can see where this is going... nearly everyone on the board buys one of the same three brands and we aren't learning anything new. You will probably want to do the same
