Fatty acid desaturase with bifunctional delta-4 and delta-5 activities. Component of a lipid metabolic pathway that catalyzes the biosynthesis of polyunsaturated fatty acids (PUFA) with preference toward n-3 substrates and Delta(4)function. (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 (7Z,10Z,13Z,16Z)-docosatetraenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (4Z,7Z,10Z,13Z,16Z)-docosapentaenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (7Z,10Z,13Z,16Z,19Z)-docosapentaenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (4Z,7Z,10Z,13Z,16Z,19Z)-docosahexaenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. The protein sequence includes a number of characteristic features of microsomal fatty acid desaturases including three histidine boxes and an N-terminal cytochrome b5 domain containing the heme-binding motif. Belongs to the fatty acid desaturase type 1 family.