Catalyzes the epimerization of trans-4-hydroxy-L-proline (t4LHyp) to cis-4-hydroxy-D-proline (c4DHyp). Is likely involved in a degradation pathway that converts t4LHyp to alpha-ketoglutarate. Can also catalyze the dehydration of trans-3-hydroxy-L-proline (t3LHyp) to Delta(1)-pyrroline-2-carboxylate (Pyr2C), albeit with 42-fold lower efficiency. Displays no proline racemase activity. trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline kcat is 40 sec(-1) for t4LHyp epimerization. kcat is 17 sec(-1) for t3LHyp dehydration. Belongs to the proline racemase family.