Catalyzes the synthesis of Und-PP-GlcNAc-ManNAcA (Lipid II), the second lipid-linked intermediate involved in enterobacterial common antigen (ECA) synthesis. N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UDP-N-acetyl-alpha-D-mannosaminouronate = beta-D-ManNAcA-(1->4)-alpha-D-GlcNAc-di-trans,octa-cis-undecaprenyl diphosphate + H(+) + UDP Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Belongs to the glycosyltransferase 26 family.