Unless you can find a professional review/lab measurement that shows the receiver's amp as stable into 4 ohms, you should not take the chance IMO. Better safe than sorry, right?
My rationale is that some "4 ohms" speakers may actually go down to 2 ohms transiently (short period of time)!
For example, the Definitive CLR3000 is supposed to be a 4 ohms speaker, but at some lower frequencies, it actually goes down to 2.1 ohms (Home Theater Magazine).