Required for neoxanthin biosynthesis, an intermediary step in abscisic acid (ABA) biosynthesis. Probably not involved directly in the enzymatic conversion of violaxanthin to neoxanthin. Cannot convert violaxanthin to neoxanthin in vitro. Required for ABA biosynthesis in response to drought stress (PubMed:17470058). Required for neoxanthin biosynthesis which is involved in photoprotection of photosystem II (PSII). Neoxanthin acts as an antioxidant within the photosystem PSII supercomplex (PubMed:17351115). Expressed in root vasculature, root hairs, leaves, trichomes, sepals, stamens, stigma, pedicels, siliques and embryo. Induced by heat stress. No visible phenotype under normal growth conditions, but mutant plants accumulate violaxanthin and completely lack neoxanthin. Plants over-expressing ABA4 have increased levels of trans-neoxanthin.