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

The last entry allows one to perform interactive merges, eg.

$ git difftool -t p4mergetool-merge <merged branch> HEAD

// Merge files now

$ git commit -m "Merged with branch <merged branch>"
$ git push