B&W speakers running on Mac 4 ohm taps?

Using a lower impedance source is not a problem. A problem would exist using 4 ohm speakers on a 8 ohm tap. THAT could damage the amp because too much current can flow through the speakers and the amp would overheat, or blow fuses.
What you are doing is fine, and may deliver a bit more snap, or sense of aliveness.

I have another question in line with this discussion. With a 6 ohm rated speaker having separate bass and m/t connectors and using a mac amp can one use both 4ohm and 8ohm of the mac in a true bi-wire configuration. Such as hooking the 4ohm to the bass unit of the speaker and the 8ohm tap separately hooked to the m/t unit of the speaker.

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.

Elizabeth

So based on your response I should use all four separate cables on either 4ohm or the 8ohm taps. The cables come as 4 individual strands not shotgun configuration.

correct. good luck! (I own B&W 805s and like your 803s)