Most of the time, it has to do with smoothing algorithms. Some of these algorithms need 4-edge-polys to achieve predictable and good looking results. Other algorithms need 4-edge-vectors or poles (that is, a vector with 4 edges coming out from it) to get good results.

It also can get better deformations to use 4-edge polys, since the polys are a bit more evenly distrubuted in that case.

McWolfe