Catalyzes the post-translational formation of 4-hydroxyproline in -Xaa-Pro-Gly- sequences in collagens and other proteins. 2-oxoglutarate + L-prolyl-[collagen] + O2 = CO2 + succinate + trans-4-hydroxy-L-prolyl-[collagen] Binds 1 Fe(2+) ion per subunit. Heterotetramer of two alpha chains and two beta chains. Exists either as a phy-1(2)/pdi-2(2) tetramer or as a phy-1/phy-2/pdi-2(2) tetramer. Belongs to the P4HA family.