Catalyzes the deformylation of 4-deoxy-4-formamido-L-arabinose-phosphoundecaprenol to 4-amino-4-deoxy-L-arabinose-phosphoundecaprenol. The modified arabinose is attached to lipid A and is required for resistance to polymyxin and cationic antimicrobial peptides. 4-deoxy-4-formamido-alpha-L-arabinopyranosyl di-trans,octa-cis-undecaprenyl phosphate + H2O = 4-amino-4-deoxy-alpha-L-arabinopyranosyl di-trans,octa-cis-undecaprenyl phosphate + formate 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 2/2. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Activated by low magnesium concentrations, via the two-component regulatory system PhoP/PhoQ. Belongs to the polysaccharide deacetylase family. ArnD deformylase subfamily.