Serine/threonine protein kinase that plays a role in a variety of different signaling pathways including cytoskeleton regulation, cell migration, growth, proliferation or cell survival. Activation by various effectors including growth factor receptors or active CDC42 and RAC1 results in a conformational change and a subsequent autophosphorylation on several serine and/or threonine residues. Phosphorylates and inactivates the protein phosphatase SSH1, leading to increased inhibitory phosphorylation of the actin binding/depolymerizing factor cofilin. Decreased cofilin activity may lead to stabilization of actin filaments. Phosphorylates LIMK1, a kinase that also inhibits the activity of cofilin. Phosphorylates integrin beta5/ITGB5 and thus regulates cell motility. Phosphorylates ARHGEF2 and activates the downstream target RHOA that plays a role in the regulation of assembly of focal adhesions and actin stress fibers. Stimulates cell survival by phosphorylating the BCL2 antagonist of cell death BAD. Alternatively, inhibits apoptosis by preventing caspase-8 binding to death domain receptors in a kinase independent manner. Plays a role in cell-cycle progression by controlling levels of the cell-cycle regulatory protein CDKN1A and by phosphorylating RAN. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Inhibited by INKA1; which inhibits the serine/threonine-protein kinase activity by binding PAK4 in a substrate-like manner. Interacts tightly with GTP-bound but not GDP-bound CDC42/p21 and weakly with RAC1 (By similarity). Interacts with FGFR2 and GRB2 (PubMed:12529371). Interacts with INKA1. Interacts with SH3RF2 (By similarity). Seems to shuttle between cytoplasmic compartments depending on the activating effector. For example, can be found on the cell periphery after activation of growth-factor or integrin-mediated signaling pathways. Autophosphorylated on serine residues when activated by CDC42/p21 (By similarity). Phosphorylated on tyrosine residues upon stimulation of FGFR2 (PubMed:12529371). Methylated by SETD6. Polyubiquitinated, leading to its proteasomal degradation. Mice die at embryonic day 11.5 probably due to a defect in the fetal heart. They show strong defects in neuronal development and axonal outgrowth. Spinal cord motor neurons and interneurons failed to differentiate and migrate to their proper position. Nervous system-specific conditional PAK4 deletion mice display growth retardation and die prematurely. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.