Based on the numbering and memory configurations (72 vs 30 pin) I would say UMC 498 VLB Chipset is the successor of the 491. But in lack of datasheets the actual differences are hard to point out.
Mr. Steunebrink points out that UMC 498F (UM8498F/8496F) supports write-back 486 processors, where the 491 is not listed. Link

My new UM498 based system is described here, and to my knowledge the board brings out the full potential of the DX4 processors, with the exception of write-back modes. I intend to do a BIOS update, to see if that will bring write-back support.