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. Inhibited by poly(L-proline). Heterotetramer of two alpha-1 chains and two beta chains (P4HB)(the beta chain is the multi-functional PDI), where P4HB plays the role of a structural subunit; this tetramer catalyzes the formation of 4-hydroxyproline in collagen. Expressed at least in brain, heart and lung. Belongs to the P4HA family.