Acts predominantly in the phot1 pathway. Involved in the leaf positioning and also in the phot2 pathway controlling the leaf flattening. Component of the network that modulates the very low-fluence response (VLFR) branch of phyA signaling. Regulates phytochrome-mediated photomorphogenesis and hypocotyl phototropism. May act by controlling auxin homeostasis. Interacts with PKS1, RPT3, PHOT1 and PHOT2. Expressed in leaves, with the strongest expression on edges of the laminas. Not found in roots. Up-regulated by white light. Increased hypocotyl growth inhibition and cotyledon unfolding responses in the very low fluence response (VLFR) mode. Reduced phototropic response. Reduced hyponasty when grown under blue light. No effect on negative root phototropism. Auxin accumulation in protoplasts. PKS1, PKS2 and/or PKS4 are essential for phototropism but not for inhibition of gravitropism under long-term blue light irradiation. Belongs to the PKS family.