Galactosyltransferase that adds one galactose residue in the beta-1-4 linkage to GlcNAc-alpha-pyrophosphate-lipid in the biosynthesis of the O-polysaccharide repeating unit of the O antigen. N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UDP-alpha-D-galactose = beta-D-Gal-(1->4)-alpha-D-GlcNAc-di-trans,octa-cis-undecaprenyl diphosphate + H(+) + UDP Divalent metal cation. Mn(2+), Ni(2+), and Pb(2+) enhance the enzyme activity. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Belongs to the glycosyltransferase 26 family.