Speaker Ohm Ratings - Need Help

[MyKill Myers]

Onkyo TX-SR705
Polk Audio CS1 Center (Rated 8 Ohms)
Polk Audio Monitor 50 Fronts (Rated 8 Ohms)
(4) Samsung Surrounds (Rated 4 Ohms)

I upgraded my Samsung HT-AS720ST in the last 12 to 18 months with the Onkyo and Polk equipment noted above but kept (4) of the Samsung speakers bundled in the HTIB to save $$$ in moving to 7.1 HD Audio

My questions/concerns are with the mix/match I've got going on in ohm ratings. I plan on upgrading my surrounds to the PA FXi A4 (Rated 8 Ohms) but for now in my receiver setup settings, I only have the option to set the speakers at 4 ohms or 6 ohms, not 8 ohms and I believe I have the setting set at 4 ohms.

Can someone give me or direct me to a quick speaker ohm rating 101 and let me know if setting the receiver speaker setting menu at 4 ohms is the right thing to do? Am I hurting my better PA 8 ohm speakers on my front stage? If I had (4) 8 ohm surrounds in place, would an 8 ohm setting magically appear in the Onkyo receiver for me to switch it to?

Thanks for any help in advance.