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 (PubMed:10588879). 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 (PubMed:10588879). Has no activity toward 9-cis and 13-cis retinoic acid stereoisomers. May play a role in the oxidative metabolism of xenobiotics such as tazarotenic acid (By similarity). all-trans-retinoate + O2 + reduced [NADPH--hemoprotein reductase] = all-trans-(4S)-hydroxyretinoate + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Expressed at stage 4 in the ectoderm, stage 5-7 in the nascent notochord and at stage 7 its expression decreases in the anterior part of the embryo. From stage 7-10 its expression is restricted to the dorsal folds of the neural tube and to rhombomere 2. At stage 10, it is expressed in the lateral plate endoderm and in the tail bud and by stage 11/12 it disappears in the neural tube, followed by a confined expression at stage 12 to dorsal neural tube and at stage 15 an increasing expression in the ectoderm. By retinoic acid. Belongs to the cytochrome P450 family.