You could probably get away with it.. though because of the impedance mismatch you would totally screw up the tonal balance of your speakers.
I would NOT use both 4 ohm and 8 ohm taps to biwire your speakers, it would be an aural mess.
I would NOT use both 4 ohm and 8 ohm taps to biwire your speakers, it would be an aural mess.

