4 ohms or not

I have just picked up a 2nd hand pair of RM series 2 wall mount speakers. and the label on the back of the speakers has nothing about the rated Ohms of the speakers, however someone has hand written on them "4 Ohms"
could anyone clarify this for me. The serial number of the speaker is..."RMSS 28374"

If the speakers are 4 ohms are they safe to run with a new RX-V 661 Yamaha amp that supports 6 and 8 Ohm only? What damage could it do? I intend to use the speakers as rear centers or surround L/R's.

Thanks in advance for feed back

Tim
Post edited by whatsthatbeepin on