I'm really only running two sets of speaker wire to the same binding post from two different taps, 8 and 4 ohm.
Perhaps I'm confused but it sounds to me like you would be shorting out 1/2 of your output transformer, which would be a major problem. Don't do this unless you are positive you are doing it correctly or you could do some serious damage to the amp.
I like the other idea of using 1/2 the cable and leaving the others disconnected.