Involved in the degradation of 4-hydroxybenzoate (4HB) via the protocatechuate (PCA) 2,3-cleavage pathway. Catalyzes the conversion of 4HB into 2-hydroxypenta-2,4-dienoate (HPD). It is highly specific for 4-hydroxybenzoate, and is able to utilize both NADH and NADPH as electron donors at approximately equal rates. 4-hydroxybenzoate + H(+) + NADH + O2 = 3,4-dihydroxybenzoate + H2O + NAD(+) 4-hydroxybenzoate + H(+) + NADPH + O2 = 3,4-dihydroxybenzoate + H2O + NADP(+) Binds 1 FAD per subunit. Belongs to the aromatic-ring hydroxylase family.