Acts as a sphingolipid delta(4)-desaturase. an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acylsphing-4-enine + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; sphingolipid metabolism. Belongs to the fatty acid desaturase type 1 family. DEGS subfamily.