Frank Durda IV had developed a ROM C which he proposed to have Tandy issue as an update. It added functionality and fixed some bugs. That ROM was not released.
When Frank left Tandy and started M.A.D. sofware, he offered a ROM C replacement for U.S. Model 4’s, called the XROM. Model 4 NGA systems would use this ROM. Other Model 4 systems could use an expanded version called the XDROM, which had some diagnostic code added as well.
A detailed disassembly of that ROM is now available on this sites’ Model 4 ROM Explained- XDROM page.