Lots of good stuff in this (old) thread, thanks to everyone who input.
I just thought I would chime in with my (very limited) 4-channel experience. My operations are purely stealth, so this is very different to all the advice given previously which seems to relate to open taping. I ran a 4-channel stealth set up last week, B3's (omnis) on shoulders in rough A-B, so approx 400mm separation, and CK930's (cards) in hat-mount above temples, pointing inwards slightly, both mic pairs going into an R-26. So visually, face-on, the mics were in a roughly triangular arrangement. I recorded at the front of the stage, centre. Purists might argue this is not a good place to tape, but I had a close-up and direct line of sight to all instruments on-stage (small stage at that).
I have to say that the results are better than I could ever have hoped. The mix of the omni's and cards (50:50) has brought out the best of both sets of mics. For this set-up at least, the matrix is noticeably better than either single source. The B3's picked up the kick drum well, but the snare drum hits were better represented on the CK930's. I would normally EQ my B3 recordings to brighten things up a tad, but when mixed with the CK930's with the 10k bump I found this was not needed as the two mics seem to compliment each other very well. Added bonus is that stereo imaging is great. No phasing issues of any kind, and mixing 2 x time and machine-synched files was a total breeze.
Wherever possible, this is going to be my stealth set up of choice from now on. But does anybody else have any good stealth 4-channel suggestions they can make, possibly some clever way of adapting my set up to provide even better results?