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-3 chains and two beta chains (the beta chain is the multi-functional PDI). N-glycosylation plays no role in the catalytic activity. Belongs to the P4HA family.