To squash the last 4 commits into one:
git rebase -i HEAD~4
Change the bottom 3 commits from pick to f or fixup and save the file.
Keep or change the top commit message to whatever you want.
Then force push to the remote branch:
git push -f
If you just want to change the last commit and e.g. add a file to it you can use the shortcut:
git commit --amend --no-edit
# instead of
git rebase -i HEAD~2
# ...