I posed this 4-mic question to Doug Oade a while back (in regards to a possible R4 purchase) and this was his response (of course, ymmv):
Yes, a four mic mix can be done with a few techniques and a lot of effort in setup and mixing. For example, 90° hypers X-Y with 8 foot spread omnis, about 20dB down, is one of a very few good techniques, but only if you can get good alignment. That is the trouble. Almost all 4 mic mixes destroy transient response, sounding somehow dull or lifeless at their very best. It also causes massive image smear in most cases. Selecting the right set of polar patterns, spacing and angles can make it possible to mix an omni, or pair of omnis, without severe degradation. I have never heard 4 directional mics mixed that was anything other than bad. It must be a main pair with omni flanking mics to open up the sound and flesh out the low end. Most folks probably have low-end to mid-range systems that cannot recreate the recorded space accurately, and hence listen only for flavors which does change with more mics, but accurate 3D space reproduction suffers dramatically....peace...Doug