Required for hydroxylation of C-4 in the sphingoid moiety of ceramide. Catalyzes the conversion of sphinganine to phytosphingosine in sphingolipid biosynthesis. Involved in the response to syringomycin. 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 + sphinganine = (4R)-hydroxysphinganine + 2 Fe(III)-[cytochrome b5] + H2O an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acyl-(4R)-4-hydroxysphinganine + 2 Fe(III)-[cytochrome b5] + H2O an N-acyleicosasphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = 2 Fe(III)-[cytochrome b5] + H2O + N-acyl-4-hydroxyeicosasphinganine Membrane lipid metabolism; sphingolipid biosynthesis. Present with 54300 molecules/cell in log phase SD medium. Belongs to the sterol desaturase family.