The function of ENSG00000133835 (HSD17B4, hydroxysteroid 17-beta dehydrogenase 4) is as follows. Bifunctional enzyme acting on the peroxisomal fatty acid beta-oxidation pathway. Catalyzes two of the four reactions in fatty acid degradation: hydration of 2-enoyl-CoA (trans-2-enoyl-CoA) to produce (3R)-3-hydroxyacyl-CoA, and dehydrogenation of (3R)-3- hydroxyacyl-CoA to produce 3-ketoacyl-CoA (3-oxoacyl-CoA), which is further metabolized by SCPx. Can use straight-chain and branched-chain fatty acids, as well as bile acid intermediates as substrates. May play a role in peroxisomal beta-oxidation step in polyunsaturated fatty acids (PUFAs) biosynthesis. Possibly regulates systemic levels of docosahexaenoic acid (DHA, C22:6n-3) through a process involving endoplasmic reticulum desaturation and elongation of alpha-linolenic acid (ALA, C18:3n-3) to form tetracosahexaenoic acid (THA, C24:6n-3), which is then beta-oxidized to DHA in peroxisomes. {ECO:0000269|PubMed:10671535, ECO:0000269|PubMed:10706581, ECO:0000269|PubMed:15060085, ECO:0000269|PubMed:8902629, ECO:0000269|PubMed:9089413, ECO:0000269|PubMed:9482850, ECO:0000303|PubMed:10903473, ECO:0000303|PubMed:35951481, ECO:0000303|PubMed:7685190, ECO:0000305|PubMed:11734571}.