Catalyzes the conversion of UDP-4-keto-arabinose (UDP-Ara4O) to UDP-4-amino-4-deoxy-L-arabinose (UDP-L-Ara4N). The modified arabinose is attached to lipid A and is required for resistance to polymyxin and cationic antimicrobial peptides. 2-oxoglutarate + UDP-4-amino-4-deoxy-beta-L-arabinose = L-glutamate + UDP-beta-L-threo-pentopyranos-4-ulose Nucleotide-sugar biosynthesis; UDP-4-deoxy-4-formamido-beta-L-arabinose biosynthesis; UDP-4-deoxy-4-formamido-beta-L-arabinose from UDP-alpha-D-glucuronate: step 2/3. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homodimer. Activated by low magnesium concentrations, via the two-component regulatory system PhoP/PhoQ. Belongs to the DegT/DnrJ/EryC1 family. ArnB subfamily.