1.4.x
fix a bug using at to setitem will raise InvalidIndexError: df.at[slice, item] = array_or_other if item existed
> * Why is this PR is against `1.4.x` branch and not to `main`?

I don't think you can change that after the PR is opened? so closing this one.

@agodbug please reopen PR against main. (after considering the points raised in https://github.com/pandas-dev/pandas/pull/48120#pullrequestreview-1075270081)
