The propfan (or UDF - Unducted Fan) used about half the fuel of a CFM-56 which it was targetted to compete/replace. It would have less of an advantage over newer engines, and the big problem is the noise of a UDF. Not stage III (I believe) and certainly not stage IV, and one more thing - there is no rear-engined airframe in production today on which you could hang them. (I think the 717 is finished ?)