Catalyzes the incorporation of an atom of dioxygen into p-hydroxybenzoate (p-OHB) to form 3,4-dihydroxybenzoate (3,4DOHB). The reaction occurs in two parts: reduction of the flavin adenine dinucleotide (FAD) in the enzyme by reduced nicotinamide adenine dinucleotide phosphate (NADPH) in response to binding p-hydroxybenzoate to the enzyme and oxidation of reduced FAD with oxygen to form a hydroperoxide, which then oxygenates p-hydroxybenzoate. 4-hydroxybenzoate + H(+) + NADPH + O2 = 3,4-dihydroxybenzoate + H2O + NADP(+) Binds 1 FAD per subunit. kcat is 55 sec(-1) for hydroxylase activity (PubMed:9578477, PubMed:9694855). kcat is 55 sec(-1) for hydroxylase activity (at 25 degrees Celsius) (PubMed:1459126). kcat is 55 sec(-1) for hydroxylase activity (at pH 8) (PubMed:10025942, PubMed:10493859). kcat is 9 sec(-1) for hydroxylase activity (at 6 degrees Celsius) (PubMed:1459126). Aromatic compound metabolism; benzoate degradation via hydroxylation; 3,4-dihydroxybenzoate from benzoate: step 2/2. Homodimer. Belongs to the aromatic-ring hydroxylase family.