Transfers N-acetylgalactosamine (GalNAc) from UDP-GalNAc to N-acetylglucosamine-beta-benzyl with a beta-1,4-linkage to form N,N'- diacetyllactosediamine, GalNAc-beta-1,4-GlcNAc structures in N-linked glycans and probably O-linked glycans. Mediates the N,N'- diacetyllactosediamine formation on gastric mucosa. . This is the function of B4GALNT3 (beta-1,4-N-acetyl-galactosaminyltransferase 3, ENSG00000139044).