You can try both, it won't hurt anything. You will find that the bass is tighter and overall sound is cleaner, deader, softer on 4 ohm tap, which has more leverage to convert voltage into current. 8 ohm tap is more open and alive sounding, and a few dB louder, but bass will be "warmer" (looser and less controlled) and probably more pleasant tubular colorings in the midrange too.
My preference is for the highest tap that the speakers will allow, but rock and bass heavy music will prefer more damping from the lower tap.