Catalyzes the o-hydroxylation of 3-hydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione (3-HSA) to 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione (3,4-DHSA) in the catabolism of cholesterol. 3-hydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione + FMNH2 + O2 = 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione + FMN + H(+) + H2O Lipid metabolism; steroid biosynthesis. Homotetramer. HsaAB monooxygenase consists of an oxygenase component HsaA and a reductase component HsaB. Belongs to the HpaH/HsaA monooxygenase family.