Removes the 4'-phosphate moiety from lipid IV(A) (a tetraacylated precursor of lipid A) and from pentaacylated lipid A, but not from hexaacylated lipid A (as is found in E.coli). Does not dephosphorylate phosphatidic acid, phosphatidylglycerophosphate, or the 1-phosphate group of lipid A and lipid A precursors. Its expression in E.coli confers resistance to the cationic antimicrobial peptide (CAMP) polymyxin B (PubMed:16467300). Plays a critical role in the ability of the bacteria to avoid the host's innate immune system, especially the bactericidal action of CAMPs, although whether it is CAMP-sensitivity or increased sensitivity to the immune system is not clear (PubMed:17360489). Bacterial outer membrane biogenesis; LPS lipid A biosynthesis. Activity depends on msbA function when expressed in E.coli, strongly suggesting this protein acts in the periplasm (MsbA flips the lipopolysaccharide precursor across the cell inner membrane). Lipid A retains the 4'-phosphate group (giving bi-phosphorylated lipid A) and a 3'-hydroxyacyl chain not usually seen in wild-type cells. Cells grow more slowly in liquid culture and are more sensitive to the CAMP polymyxin B. Bacteria are avirulent upon infection of C57BL/6 mice, the natural host of this bacterium; they do not colonize mouse spleen. Upon mouse intraperitoneal injection induces pro-inflammatory cytokine IL-6 and monocyte chemoattractant protein 1 and keratinocyte-derived chemokine, two potent chemoattractants; bacterial viability decreases about 100-fold. In Francisella lipid A is mono-phosphorylated at position 1- and tetraacylated, it lacks the 4'-phosphate residue found in E.coli (PubMed:17263332, PubMed:17360489). More than 90% of lipid A is in a free form that lacks core oligosaccharide and O-antigen (PubMed:17360489). Belongs to the lipid A LpxF 4'-phosphatase family.