Involved in the degradation of para-nitrophenol (4-NP). Catalyzes oxidation of 4-nitrophenol (4-NP) at position 4 with concomitant removal of the nitro group as nitrite and production of para-benzoquinone. 4-nitrophenol + H(+) + NADPH + O2 = 1,4-benzoquinone + H2O + NADP(+) + nitrite Xenobiotic degradation; 4-nitrophenol degradation. Monomer. Cells lacking this gene are not able to grow on 4-NP. Belongs to the PheA/TfdB FAD monooxygenase family.