Removes the 4'-phosphate moiety from lipid IV(A) (a tetraacylated precursor of lipid A). Bacterial outer membrane biogenesis; LPS lipid A biosynthesis. In R.leguminosarum strain 3841 lipid A lacks phosphate groups, and both tetra- and pentaacylated species exist; the same is probably true of this strain. Belongs to the lipid A LpxF 4'-phosphatase family.