Come again, please? You lost me.
You're saying if I have a sidebar in this default theme (who doesn't?), my footer automatically defaults to a 3-wide footer instead of normal 4? That can't be right. It looks off.
So, the only way to get a normal, clean, evenly spaced footer with 4 widgets spread out is to never use a sidebar in this theme? Is that what you are saying?
The parent theme has a sidebar AND footer with 4 widgets, just fine.
But when you create a child theme, and make no changes, those 4 widgets change alignment with the same sidebar. That can't be correct behavior, it's not consistent.