Catalyzes the post-translational formation of 4-hydroxyproline in -Xaa-Pro-Gly- sequences in proline-rich peptide sequences of plant glycoproteins and other proteins. Hydroxyprolines are important constituent of many plant cell wall glycoproteins such as extensins, hydroxyproline-rich glycoproteins, lectins and arabinogalactan proteins. Possesses high affinity for leucine-rich repeat and proline-rich extensins of root cell walls that are essential for root hair development. Hydroxyprolines define the subsequent O-glycosylation sites by arabinosyltransferases which elongate the O-arabinosides on extensins. Has low affinity for the substrates tested in vitro. 2-oxoglutarate + L-prolyl-[collagen] + O2 = CO2 + succinate + trans-4-hydroxy-L-prolyl-[collagen] Binds 1 Fe(2+) ion per subunit. Predominantly localized in the endoplasmic reticulum. Expressed in epidermal root hair cells (trichoblasts). Reduced root hair length and content of hydroxyproline in root cell walls. Belongs to the P4HA family. Truncated N-terminus.