Catalyzes the post-translational formation of 4-hydroxyproline in -Xaa-Pro-Gly- sequences in proline-rich peptide sequences of plant glycoproteins and other proteins. Hydroxylates preferentially prolines in second positions in the -Pro-Pro-Gly-triplets. Hydroxyprolines are important constituent of many plant cell wall glycoproteins such as extensins, hydroxyproline-rich glycoproteins, lectins and arabinogalactan proteins. Can hydroxylate collagen-like peptides and hypoxia-inducible transcription factor peptides. 2-oxoglutarate + L-prolyl-[collagen] + O2 = CO2 + succinate + trans-4-hydroxy-L-prolyl-[collagen] Binds 1 Fe(2+) ion per subunit. By hypoxia. Plants over-expressing P4H1 show absence of trichome on leaf and petal surfaces, increased root hair and reduction in seed size. Belongs to the P4HA family.