Catalyzes the epimerization of trans-4-hydroxy-L-proline (t4LHyp) to cis-4-hydroxy-D-proline (c4DHyp). Is involved in a degradation pathway that converts t4LHyp to alpha-ketoglutarate, which allows R.sphaeroides to grow on t4LHyp as a sole carbon source. Displays no proline racemase activity. trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline Is up-regulated when the bacterium is grown on t4LHyp as sole carbon source. Cells lacking this gene loss their ability to grow on t4LHyp as a sole carbon source. Belongs to the proline racemase family.