Sphingolipid-delta-4-desaturase required for the biosynthesis of delta-4-unsaturated sphingolipids and derivatives. May be required for the biosynthesis of glucosylceramides. an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acylsphing-4-enine + 2 Fe(III)-[cytochrome b5] + 2 H2O Specifically expressed in flowers. No visible phenotype under normal growth conditions. Belongs to the fatty acid desaturase type 1 family. DEGS subfamily.