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, which would allow P.denitrificans to grow on t4LHyp as a sole carbon source (PubMed:24980702). Also seems to be involved in an alternative catabolic pathway that degrades trans-4-hydroxy-L-proline betaine (tHyp-B) to alpha-ketoglutarate; this pathway would permit the utilization of tHyp-B as a sole carbon and nitrogen source (PubMed:24056934). trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline kcat is 15 sec(-1) (PubMed:24056934). kcat is 16 sec(-1) (PubMed:24980702). Homodimer. Is up-regulated when the bacterium is grown on t4LHyp as sole carbon source. Cells lacking both hpbD and hypF are unable to utilize tHyp-B or t4LHyp as sole carbon source. Belongs to the proline racemase family.