Delta(4)-fatty-acid desaturase which introduces a double bond at the 4-position in the long-chain base (LCB) of ceramides. Required for sphingosine biosynthesis. 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. Results in the dramatic reduction in the endogenous levels of sphingosine. Belongs to the fatty acid desaturase type 1 family. DEGS subfamily.