Here’s a brief summary of the official documentation:

git config --global diff.tool bc3
git config --global difftool.bc3.path "c:/program files/beyond compare/bcomp.exe"
git config --global merge.tool bc3
git config --global mergetool.bc3.path "c:/program files/beyond compare/bcomp.exe"

Don’t ask me why BC4 has the shortcut called BC3 still. Go figure. And happy merging! 🙂