It has been experimentally seen that flexural moduli obtained by three-point and four-point bending tests are different for the same specimen. The slope of the load-displacement curve increases as load increases in both tests, showing an apparent stiffening of the specimen. The present work analyses the effect of the variation of the support span and the load span caused by the variation of the contact zone between the specimen and support and load rollers. These effects have been analysed by classical beam theory without taking into account shear effects. Experimental differences greater than 5% for the bending modulus have been obtained for the same specimen tested in three-point and four-point bending, using two specimens of different carbon/epoxy composite materials. After corrections based on the analysis developed in this work, the relative differences between three-point and four-point moduli for the same specimen were under 1% for both specimens.