I've experimented a bit and am on the beginning of a learning curve.
What I've done so far involves more mics than are needed, but they give me choices and help me to mitigate my lack of experience somewhat. You have to balance your own needs for more gear in the performers' faces with their own needs for an appropriate performance setting, particularly if this is a live concert.
Anyway, here's what I've done with some useful results. Assume the musicians are in a semicircle or a circle. Most everything uses overheads. This uses seven mics, one for most of the signal and the others provide a little imagery, soundstage depth and color.
One omni LDC in the center of the circle or semicircle, upside down and overhead at about six or seven feet high. I use a SP C3 and like it quite a lot. This gets me the strongest overall signal and tends to miss nothing.
One cardioid LDC at either side of the quartet (say, at four and eight o'clock) and a foot behind the musicians, about five feet high, pointed across the circle (2 & 10 c'clock) and slightly down (10 degrees) at the other side. I use what I have - SP B1 works fine for this.
One SDC overhead, pointing down, suspended 2' or 3' over the violin / viola. Neumann KM185. One SDC overhead over the cello player's right shoulder. Also a KM185.
A couple SDC ambient mics at a significant distance, X-Y and up high. MXL 603s.
I like to keep mics away from the instruments a bit, especially the cello. There's less harshness, less bow noise and more complexity to the sound. If you have time to experiment, try to pick up a signal from behind the cello - down pretty low, like a couple of feet off the gound and a couple of feet behind the player. There's a lot of sound going on there.
This will give you a palette of interesting stuff to work with.
You can also do this with one omni or a couple of LDC mics, or a combination of 4 LDC and SDC mics - you don't need a truckload to get a good sound.
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