The main difference between 3 point and 4 point bend tests is the area in which the maximum bend stress occurs. In 3 point bend tests it is under the loading nose while for 4 point bend tests it is distributed in a wider area between the loading points. The 3 point test best applies when the material is homogeneous such as in the case of plastic materials. A 4 point test tends to be the best choice when the material is non-homogeneous such as some types of composites.
The stress concentration of a three point test is small and concentrated under the center of the loading point, whereas the stress concentration of a four point test is over a larger region, avoiding premature failure.