The function of GALNT4 (polypeptide N-acetylgalactosaminyltransferase 4, ENSG00000257594) is as follows. Catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. Has a highest activity toward Muc7, EA2 and Muc2, with a lowest activity than GALNT2. Glycosylates 'Thr-57' of SELPLG. {ECO:0000269|PubMed:10984485, ECO:0000269|PubMed:16207894, ECO:0000269|PubMed:29208955, ECO:0000269|PubMed:9804815}.