Four and Six Ohm speakers, help on settings?

[Automission]

My speaker set up consists of two rears and a centre speaker rated at six ohms, the front two towers are rated at four ohms. On my european Onkyo 606, I can change the speaker impedance to either four or six ohms. I've had it on Six ohms, but would four be better or worse?

Wouldn't four ohms over power and blow up the six ohm rated speakers?