At 4 inches apart, 4 of my 8 taps would be 12 inches from dead center of the 1st, to dead center of the last. That leaves 7 inches, divided by 2, would give 3.5" the outside of the 1st and last tap.
At 3 inches apart, 5 taps would fit in the same first to last span of 12 inches.
At 3.5...