Bifunctional enzyme which acts as both a sphingolipid delta(4)-desaturase and a sphingolipid C4-monooxygenase. a dihydroceramide + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a phytoceramide + 2 Fe(III)-[cytochrome b5] + H2O an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acylsphing-4-enine + 2 Fe(III)-[cytochrome b5] + 2 H2O 2 Fe(II)-[cytochrome b5] + 2 H(+) + N-octanoylsphinganine + O2 = 2 Fe(III)-[cytochrome b5] + H2O + N-octanoyl-4-hydroxysphinganine an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acyl-(4R)-4-hydroxysphinganine + 2 Fe(III)-[cytochrome b5] + H2O Lipid metabolism; sphingolipid metabolism. Belongs to the fatty acid desaturase type 1 family. DEGS subfamily.