Can function as a Delta(5) fatty acid desaturase and behaves as a (8-3) desaturase. Introduces a double bond in the fatty acid chain 5 carbons away from carboxy terminal to biosynthesize polyunsaturated fatty acids (PUFAs) endogenously (PUFAs are essential for membrane structure and many cellular and physiological processes). Acts on a variety of substrates such as dihomo-gamma-linoleoyl-CoA ((8Z,11Z,14Z)-eicosatrienoyl-CoA, 20:3n-6) to generate arachidonoyl-CoA ((5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA, 20:4n-6). Also acts on a number of other substrates, including fatty acids that do not contain a double bond at the 8 position like (11Z,14Z,17Z)-eicosatrienoyl-CoA (20:3n-3) to produce (5Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA (20:4n-3) (PubMed:9845325, PubMed:9917342, PubMed:11972048, PubMed:26806391). Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates (By similarity). (11Z,14Z)-eicosadienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z,11Z,14Z)-eicosatrienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (11Z,14Z,17Z)-eicosatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (8Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (8Z,11Z,14Z)-eicosatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. The fat-4 mutants lack detectable Delta(5)-unsaturated fatty acids. HPO-19 and T05H4.4 are cytochrome b5 reductases required for PUFA desaturation in Caenorhabditis elegans. HPO-19 knockdown or mutation alters FAT-4 desaturase activity. Belongs to the fatty acid desaturase type 1 family.