I rename the KiCad 4 program folder from KiCad to KiCad4, and readjust just some environments values to have it work as normal. Then install KiCad5 with some adjust in environments (trial and error). Now, I can run either Kicad4 and/or 5 (sametime too) on my machine without changing the environments. I think my trick also because I specify all my file path (data sheet) as relative path in my customized libraries, and in the .pro file also use relative path like LibDir, and other …Dir variable (Not environments one). - In fact, I can open the same project from either version 4 or 5 (Don’t try open at the same time - i is a bad idea)