You can upgrade FSR 3.1 games to FSR 4 with manual DLL swapping — GitHub community discovers FSR swapping works similar to DLSS upgrades
by editors@tomshardware.com (Aaron Klotz) from Latest from Tom's Hardware on (#6ZNED)
GitHub user has discovered that DLL swapping works with FSR 4 in FSR 3.1 games. Allowing FSR 3.1 games to be upgraded to FSR 4 merely by replacing a few DLLs in the game's files.