In vitro, catalyzes the epimerization of trans-4-hydroxy-L-proline (t4LHyp) to cis-4-hydroxy-D-proline (c4DHyp) and that of trans-3-hydroxy-L-proline (t3LHyp) to cis-3-hydroxy-D-proline (c3DHyp), albeit with very low efficiency. The physiological substrate may be different (PubMed:24980702). Displays neither proline racemase activity nor t3LHyp dehydratase activity (PubMed:17849014, PubMed:24980702). trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline kcat is 0.082 sec(-1) for t4LHyp epimerization. kcat is 0.085 sec(-1) for t3LHyp epimerization. Homotetramer. Belongs to the proline racemase family.