Contains FAD. The enzyme catalyses the first step in a degradation pathway for 4-nitrophenol, the oxidation of 4-nitrophenol at position 4 with the concomitant removal of the nitro group as nitrite. The enzyme from the bacterium Pseudomonas sp. WBC-3 also catalyses EC 1.14.13.166, 4-nitrocatechol 4-monooxygenase.