Each tap is a secondary winding on the output transformer. The primary voltage is transferred to each of the 8-ohm and 4-ohm secondary windings equally. The impedance is set by the turns ratio of wires between the primary and the 4 ohm tap, and the primary and the 8 ohm tap. The relationship is the square root of the impedance so to get an 4 ohm impedance, you wind 1.44 times as much wire to the 4-ohm tap (square root of 2) as you do for the 8-ohm tap. This will set both the voltages the impedances the speakers the speaker sees when connected.