Hi @JamesHallam ,
Thanks for the update.
If you reinstall 4.0, the R packages from 3.4 will still be in "~/Library/R/3.4/library".
What I usually do is upgrade my version of R, but before I delete the old version folder, I save the names of the packages from the old version, and reinstall the packages in the folder for the new version.
oldpkgs <- installed.packages(lib.loc = "~/Library/R/3.4/library")
install.packages(oldpkgs[, 1], lib = "~/Library/R/4.0/library")
After this, it's safe to delete the old folder ~/Library/R/3.4
HTH