Catalyzes the transfer of 4-deoxy-4-formamido-L-arabinose from UDP to undecaprenyl phosphate. The modified arabinose is attached to lipid A and is required for resistance to polymyxin and cationic antimicrobial peptides. Plays an important role in pathogenesis by providing resistance to antimicrobial peptides within macrophages or at other anatomic sites encountered during infection. di-trans,octa-cis-undecaprenyl phosphate + UDP-4-deoxy-4-formamido-beta-L-arabinose = 4-deoxy-4-formamido-alpha-L-arabinopyranosyl di-trans,octa-cis-undecaprenyl phosphate + UDP Glycolipid biosynthesis; 4-amino-4-deoxy-alpha-L-arabinose undecaprenyl phosphate biosynthesis; 4-amino-4-deoxy-alpha-L-arabinose undecaprenyl phosphate from UDP-4-deoxy-4-formamido-beta-L-arabinose and undecaprenyl phosphate: step 1/2. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Induced by BasR. Belongs to the glycosyltransferase 2 family.