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. 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. Belongs to the glycosyltransferase 2 family.