If someone wanted to use P4Merge along with Git, this is – mostly likely – the fastest method.
Open global Git configuration file (C:\Users<>\.gitconfig) and add (or modify) the following sections:

[merge]
tool = p4mergetool
[mergetool "p4mergetool"]
cmd = \"C:\\Program Files\\Perforce\\p4merge.exe\" \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"
[diff]
tool = p4mergetool
[difftool "p4mergetool"]
cmd = \"C:\\Program Files\\Perforce\\p4merge.exe\" \"$LOCAL\" \"$REMOTE\"