I agree that the 8 ohm output should be fine. I also think that the 4 ohm output would be equally fine. Mathematically, if the drivers are 8 ohms (which they are, nominally), then 6 of them work out to 5.3 ohms nominal. Using 5.3 ohms, the 4 ohm output is a closer match.
However - you knew there had to be one, right? - nominal impedance is a very fluid thing, as EdBass alludes to. Many people strongly believe that a lower impedance load is safer for the amp than a higher one, if there must be a mismatch, in which case the 8 ohm setting would be safer. I am of the camp that holds that a mismatch of up to 100% is nothing to worry about.