Activates CDC42, a member of the Ras-like family of Rho- and Rac proteins, by exchanging bound GDP for free GTP. Plays a role in regulating the actin cytoskeleton and cell shape. Activates MAPK8 (By similarity). . This is the function of FGD4 (FYVE, RhoGEF and PH domain containing 4, ENSG00000139132).