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). Highly expressed in placenta, liver and fetal skin. Weakly expressed in fetal epiphyseal cartilage, fetal liver, fibroblast, lung and skeletal muscle. Expressed also in fibrous cap of carotid atherosclerotic lesions. N-glycosylation plays no role in the catalytic activity. Belongs to the P4HA family.