Hi Ed,
If you end up using that setup regularly, and aren't averse to EQ, you might play around with EQing just the rear-facing omni channel with a curve resembling the equal loudness curve (boosted bass, deeply scooped mids with maximum reduction of the upper mids, boosted highs). That helps the omni contribute what it's best at (bass extension down low + rear facing ambient air and openess up top) while keeping it out of the way of the MK4 stereo pair through the midrange, which is where the addition of the omni tends to collapse image width and sometimes muddy things up somewhat. Also helps to minimize the listener's attention being pulled toward the audience members behind the recording position acting up or talking loudly, without overall reduction of omni level. That way, you may find you are able to dial in more omni level without problems if you want to do so. Once you find a general curve that works, you can apply the same curve to the rear facing omni each time, only tweaking it further if necessary. I find this can be quite useful with my own rear facing ambiance mics.