No, hopefully you won't have to re-install the game. If you have not used MultiSH4 v1.5, then your Save folder is usually in "C:\Users \UserName \Documents \SH4", where "UserName" is the name you log into your computer with, and "Documents" might be "My PC", or "My Computer" or "This PC", all dependent upon which version of Windows you are using. Usually, the Windows File Manager (This PC, My Computer, etc) will have short-cuts available in the left section of the Window, and you could just click on the "Documents" folder short-cut icon, and look for an "SH4" folder in there. Open that folder, and delete the "Data" folder below that. Start the SH4 game again, and it will recreate the Save folder, and bring in all the new mod parts it needs.
Another thing to check is the use of LAA or some other applet that can set the "Large Address Aware" bits on the SH4.exe file. The file has to be write-enabled though first, so go into the game folder, find the "Sh4.exe" file, right-click on it, and choose "Properties" with a left-click. That will usually open a small window to a "General" tab. In the lower portion of that is "Attributes", and there will be a little square tick box for "Read-only". Be sure that is empty, then you can run LAA and flip the necessary bits for the game to address more of your computer's RAM. After you have activated the FotRSU mod, there should be a "01_FotRSU_Support_TOC.bat" that you can double-click on to start the "01_FotRSU_TableOfContents.pdf" that is in the Support folder. It has links to other help files in the Support folder.
