your best bet is to contact Onkyo and find out, or read the owners manual and see if it can handle a 4 ohm load.

Do you already own the M80's? If not, and your Onkyo can only handle 8 ohms, then the M60's aren't exactly a compromise. The difference between the two is the M80 can play louder, and is brighter and more forward. It's not a secret that many, many people prefer the sound of the M60.