Yep, and that is why a receiver rated for 6ohm speakers has some specs listed for 4ohms.. Since 6ohm speakers may vary from 4ohms to 16ohms. (or even wider) But if it were designed for 4ohm speakers it has to support even lower ohmage. (For instance the Onkyo receivers that support 4ohm speakers list specs down to 3ohms)
In any case you probably won't have any problems unless you turn it way up, and even then it should only trip the 'protection circuit'. But, in any case if something does go really wrong, they always have the out of saying "It's your fault because you hooked up speakers that aren't supported"
