Functions as downstream effector of Rho-related GTP binding proteins of the 'Rho of Plants' (ROPs) family. Participates in the propagation of ROP GTPase signals in specific cellular responses. Required for actin cortical microfilament assembly. Activated by ARAC4/ROP2 to promote the assembly of cortical actin microfilaments required for lobe formation and lateral expansion of pavement cells. Interaction with, and activation by ARAC4/ROP2 is inhibited by RIC1. Functions as downstream effector of ARAC11/ROP1 to promote the assembly of apical F-actin associated with vesicle accumulation in the tip of the growing pollen tube. Counteracts the ARAC11/ROP1-RIC3 pathway, which activates calcium signaling that leads to apical F-actin disassembly associated with exocytosis, to control actin dynamics and pollen tube apical growth. Downstream of ARAC11/ROP1, is involved in the growth responses to the root-colonizing endophytic fungus P.indica. Interacts with ARAC4/ROP2 and ARAC11/ROP1. Expressed in roots, leaves, stems, flowers, siliques and pollen. By the root-colonizing endophytic fungus P.indica. Increased rate of seed germination. Over-expression of RIC4 in tobacco germinating pollen induces depolarized pollen tube growth.