Technically speaking, 4 bearings is an "over-constrained mechanism". Two bearings on one rail insure motion along the rail's axis. Adding a third bearing on a second rail parallel to the first prevents rotation around the first rail/axis, thus constraining the motion completely. Adding a fourth bearing does nothing to further control the motion unless you consider a carriage that is not perfectly stiff.
Three or four bearings can be made to work fine, though misalignment when using four is less forgiving - i.e. more likely to bind- than the same misalignment with only three bearings.