The argument that the "scale speed" should be 1/4 of the speed of the full size plane seems obvious at first, but is it? A quarter scale model has (roughly?) 1/16 of the surface area of the full-scale plane and 1/64 the volume. A quarter scale plane sitting on the runway next to the full-scale version looks tiny in comparison. I suspect that a quarter-scale model flying at a quarter of the speed of the full-scale version would seem to someone on the ground to be going unreasonably fast. I don't have a solution, but it does seem that picking the linear dimension as the standard is arbitrary.