4. Project each subject's aligned FA data onto the skeleton generated in step 3, thresholding the mean FA skeleton image using the FSL script "tbss_4_prestats".

The last TBSS script carries out the final steps necessary before running the voxelwise cross-subject stats (hence the "prestats" monicker).  It sets the mean FA skeleton image at a chosen threshold, which has been standardized as 0.2.