It is also to be considered that in the case o 4-point bending, the volume under stress is bigger than the one under 3-point bending. If we consider the Weibull statistics, it is to be expected that the mechanical strength measured through 4-point bending will be lower than the one measured through 3-point bending. Weibull statistics says that the bigger the volume, the higher will be the probability to find a longer crack or flaw in general. Therefore, if you whant to have a more reliable mechanical strength from the point o view of tensile stress, I recommend that you use 4-point bending. That is important in the case o ceramic materials, which are very brittle and it is very difficult to prepare specimens for pure tensile tests. In the case of 3-point bending, the results of the modulus of rupture will be very sensitive to the finishing of the botton surface of the specimens.