I mostly use 3 and 4 as "supplements" when needed, but I tape a lot of small ensembles in not-so-crowded conditions a lot. I use a main stereo pair, then run split omnis on seperate stands sometimes. But many times, it's a main pair and one or two spot mics. I've run stereo pair plus one spot inside a piano many times. I got to know the stuff I was taping, and it was easy to spot when the piano (in a jazz mix usually) was going to be too quite (relegated to the back of the group sometimes). So, I run a pair plus one mic a fair amount (no fourth mic). Sometimes a pair, and then one omni back by the keys and bass and drumkit. That always seems to pick up a bunch of bass and warmth back there (on stage) that the main pair in front doesn't. Then, just mix in accordingly, and sometimes not at all (if it makes the mix worse).
Also, you can use EQ to "isolate" your intended target more if that helps your mix. Like in that last case, I might put that omni back there just for some bass. Then in post, I might rolloff a lot of the highs on that one mono channel before panning it and fading it in with the main pair until I get what I was looking for -- in that case maybe it was more sub-like lows. Many times I run with a main AUD back by the SBD (house snake if possible) and then go for split omnis stage lip in front to pick up the brass in a more rich way. In that case, in post I might roll off a bunch of bass on the omnis so they isolate the brass section a little more, then pan and mix in the omnis of the brass into the main AUD pair from back by the board. This way, I just use the omnis to sweeten the brass section up, and I'm relying on the main AUD pair to bring most of the bass (they pick up the whole room after all). Anyway, don't forget about the possibility of doing this -- EQ'ing each mic or pair independently. Because you can use it to your advantage sometimes (you can also use it to totally F it up real bad too, LOL). Another thing I do sometimes is to use a J-disc for one pair back farther and something else on for the other pair in closer to the sound source. One close and one far offers some nice possibilities in post. Use the close stereo pair as a base, then fade in the farther mics (that picked up more of the room) to add just the right amount of verb/wet into the mix.
I understand that many of those setups are not realistic depending on the venue, band, and your relationship with everyone. But in case you do get those opportunities I thought I would share some of my stories with the R4 on that front. So, that said, I haven't really run too many 4-channel recordings from a single stand. In those situations, I usually just go with a 2-channel setup like "normal" -- cuts down on the gear to haul around too. HOWEVER, if I were to try some form of it (4-channel on one stand), I'd try to push the second pair to the extremes I think. Like how about running something like Hypercard-ORTF plus XY for the middle channel. That might be interesting in post, you could spread the stereo imaging of the main XY pair just as much as you wanted to by mixing in the uber-wide Hyper-ORTF. Another extreme example might be j-disc omnis (that pick up the whole room) and then a much more focused tight second pair like XY-hypers at 60-70 degrees or something. Then mix the super-airy j-disc with the uber-tight XY-hypers@70 to see what kind of mixes produce different results. Start with the tighter XY pair and expand it with the j-disc pair, or vise-versa.
And of course, there's the OBVIOUS surround sound attempts. Not sure those are worth it considering some of the other things you could do can produce better options for a 2-channel stereo mixdown. But that said, in the right venue, in the right spot, with the right band, it might be pretty cool to point at least one hyper or card straight backward. In post just that one channel would give you enough to do mixdown to Dolby Digital 5.1 or something for kicks. I've never really done it (experimented a little), but I'd like to do it at least once -- super easy with a clamp to the stand too.
So, has anyone ever actually tried some of those single-stand 4-channel configs?