Assuming that P4Merge was installed in the default location on Windows, i.e 'C:\Program Files\Perforce\p4merge.exe' then the following commands will configure p4merge as the default tool for both merge and diffs in Git
Set up commands
git config --global diff.tool p4merge
git config --global difftool.p4merge.path '/Applications/p4merge.app/Contents/MacOS/p4merge'
git config --global merge.tool p4merge
git config --global mergetool.p4merge.path '/Applications/p4merge.app/Contents/MacOS/p4merge'