Whichever sounds best, don't worry, you won't hurt anything.
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I think the correct answer depends on the impedance curve of the speaker. 6 ohms is simply the nominal impedance. Without knowing the particulars of your speaker, I would say that the 4 ohm taps are the better choice, since the impedance will be lower than 8 ohms for much of the time.
As was said before, try both. It's possible you won't hear any difference. Only if the speaker's impedance drops into the 2 or 3 ohm range for a significant part of the audioband will the 4 ohm tap sound noticeably better.