Mercurial solves this 90% of the way with hg absorb: https://gregoryszorc.com/blog/2018/11/05/absorbing-commit-ch...

Its extremely close to magic. You make all the requested changes on the PR4 and type hg absorb and it'll figure out which commit in your stack each change belongs to.

I've used this to work on lots of stacks 20 commits deep in mercurial.