In vitro, catalyzes the epimerization of trans-4-hydroxy-L-proline (t4LHyp) to cis-4-hydroxy-D-proline (c4DHyp), albeit with very low efficiency. The physiological substrate may be different. Displays neither proline racemase activity nor t3LHyp dehydratase activity. trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline kcat is 0.070 sec(-1) for t4LHyp epimerization. Belongs to the proline racemase family.