A cytochrome P450 monooxygenase involved in the metabolism of all-trans retinoic acid (atRA), a signaling molecule that binds to retinoic acid receptors and regulates gene transcription. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH-ferrihemoprotein reductase). Catalyzes the hydroxylation of carbon hydrogen bonds of atRA primarily at C-4 and C-18. Has no activity toward 9-cis and 13-cis retinoic acid stereoisomers (PubMed:22020119, PubMed:9228017, PubMed:9716180). May play a role in the oxidative metabolism of xenobiotics such as tazarotenic acid (PubMed:26937021). all-trans-retinoate + O2 + reduced [NADPH--hemoprotein reductase] = all-trans-(4S)-hydroxyretinoate + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] all-trans-(4S)-hydroxyretinoate + O2 + reduced [NADPH--hemoprotein reductase] = all-trans-(4S,16)-dihydroxyretinoate + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] all-trans-retinoate + O2 + reduced [NADPH--hemoprotein reductase] = all-trans-18-hydroxyretinoate + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Expressed in most fetal and adult tissues with highest levels in adult liver, heart, pituitary gland, adrenal gland, placenta and regions of the brain (PubMed:9826557). Expressed at high levels in lung, pancreas, skin and uterus (at protein level) (PubMed:22020119). Lower expression level is detected in spleen, kidney, intestine and adipose tissue (at protein level) (PubMed:22020119). By retinoic acid. Belongs to the cytochrome P450 family.