Git - Make local HEAD the new master

Even though you don't want that old branch anymore, git really doesn't like rewriting history or discarding changes. Just revert and merge.

git branch new_master              # name current detached HEAD
git checkout master                # switch back to master
git revert --no-edit HEAD~4..HEAD  # create commits reverting back to where the history split
git merge new_master               # merge
git branch -d new_master           # don't need it anymore

Make HEAD your new local master:

$ git checkout -B master

Force-push your changes:

$ git push -f