Fatty acid desaturase that introduces a cis double bond at the 4-position in 22-carbon polyunsaturated fatty acids that contain a Delta(7) double bond, resulting in the production of delta-4 desaturated fatty acid docosahexanoic acid (DHA). Also acts on the 16-carbon polyunsaturated fatty acids. a (7Z,10Z,13Z,16Z,19Z)-docosapentaenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (4Z,7Z,10Z,13Z,16Z,19Z)-docosahexaenoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O a (7Z,10Z,13Z,16Z)-docosatetraenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (4Z,7Z,10Z,13Z,16Z)-docosapentaenoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O The cytochrome b5 heme-binding domain acts as the direct electron donor to the active site of the desaturase, and does not require an external cytochrome. Belongs to the fatty acid desaturase type 1 family.