As far as I know, it is depndant on the receiver which should be able to power speakers within a range (4-16ohms).
Which amp is it? The manual should dictate what rated speakers you can use, and the manual should be available online from the manufacturer's site.
Some amps can not power speakers that are 4 ohm, although I would have though if it states 4ohm on the receiver, this is likely the minimum that it can power. If you give me the model I'll check it out for you.