If the speaker impedance is not very flat then you might hear some coloration, in which case you'll want to use the proper tap. But first, just try the 4-ohm speaker on the 8-ohm tap. It won't hurt anything, and will likely give a bit more power.
The 4 ohm secondary tap is gray; the 8 ohm tap is white. You can leave the white secondary attached to the black primary (common) and the headphone jack, but run the gray to the speaker binding post - if it's not long enough, or was cut short in construction, you might have to splice a longer wire to it. If possible, keep the wire close to the ground wire that goes to the other terminal post, so it does not create a loop that might pick up magnetic-induced hum.
