Update for BC4 64bit: This works for Git for Windows v.2.16.2 and Beyond Compare 4 - v.4.2.4 (64bit Edition)
I manually edited the .gitconfig file located in my user root "C:\Users\MyUserName" and replaced the diff/difftool and merge/mergetool tags with
[diff]
tool = bc
[difftool "bc"]
path = 'C:/Program Files/Beyond Compare 4/BComp.exe'
[difftool "bc"]
cmd = \"C:/Program Files/Beyond Compare 4/BComp.exe\" \"$LOCAL\" \"$REMOTE\"
[difftool]
prompt = false
[merge]
tool = bc
[mergetool "bc"]
path = 'C:/Program Files/Beyond Compare 4/BComp.exe'
[mergetool "bc"]
cmd = \"C:/Program Files/Beyond Compare 4/BComp.exe\" \"$REMOTE\" \"$LOCAL\" \"$BASE\" \"$MERGED\"