How to change past commit to include a missed file?

I realize people can google and come here to find a simpler answer: What if it was just the last commit?
(OP’s question is for fixing the 4th commit back in history)

In the case you commit and realize you forgot to add some file immediately, just do:

# edited file-that-i-remember.txt
git add file-that-i-remember.txt
git commit

# realize you forgot a file
git add file-that-i-forgot.txt
git commit --amend --no-edit

Where --no-edit will keep the same commit message.

Easy peasy!