Has sphingolipid-delta-4-desaturase activity. Converts sphinganine-containing sphingolipids (such as N-acylsphinganines or dihydroceramides) into sphingolipids containing the delta-4-desaturated sphingoid base (E)-sphing-4-enine (such as N-acylsphing-4-enines or ceramides), which are required for many different functions (structural functions as well as signaling) (PubMed:11937514). Required to initiate spermatid differentiation among other signals (PubMed:9003299). Required for central spindle assembly and cytokinesis during male meiosis, may act as part of an anchoring mechanism that links membrane-bounded cellular compartments to components of the cytoskeleton (PubMed:9819060). an N-acylsphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acylsphing-4-enine + 2 Fe(III)-[cytochrome b5] + 2 H2O an N-acyleicosasphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = an N-acyleicosasphing-4-enine + 2 Fe(III)-[cytochrome b5] + 2 H2O Sphingolipid metabolism. Testes. Primary spermatocytes become mature in size but degenerate without initiating meiotic chromosome condensation in their nuclei. The mutation does not affect female fertility but leads to semi-lethality both in males and females during embryonic stages (PubMed:9003299). Homozigous mutant spermatids show a misshapen Nebenkern, which varies in size and is associated with multiple nuclei (PubMed:9819060). Belongs to the fatty acid desaturase type 1 family. DEGS subfamily.