Four or Five at a stretch is a voulentary green lane code of conduct draw up by GLASS many years ago and all of the big clubs , groups and organisations have adopted it.
The idea is that long streams/convoys of 4x4's on the lanes doesnt look very good, the anti's would have a field day taking pictures of a green lane looking like a motorway. Also now and again on a narrow lane you meat a group coming the opposite direction and if each group had say eight or ten cars in it would be a nightmare to pass, thus few cars have less impact on the lane and its surroundings.