Utilizes the flavins supplied by NphA2 to catalyze the degradation of 4-nitrophenol (4-NP) via 4-nitrocatechol (4-NC) which is used as the sole carbon, nitrogen, and energy source. Can also degrade phenol and 4-chlorophenol as rapidly as 4-NP. 4-nitrophenol + H(+) + NADH + O2 = 4-nitrocatechol + H2O + NAD(+) Partially inhibited by concentrations of FAD above 10 uM and completely inhibited by concentrations above 50 uM. Optimum pH is 8. Homotetramer. 4-nitrophenol 2-monooxygenase complex consists of an oxygenase component NphA1 and a flavin reductase component NphA2. By 4-NP in the presence of NphR. Belongs to the FADH(2)-utilizing monooxygenase family.