This is a hard problem and I usually try to avoid it, but when I have to do it I usually have them branch off each other so that I can merge the PRs in reverse order once they're approved (4 -> 3 -> 2 -> 1 -> main). The diffs are all based off the previous branch in the "stack" so they aren't duplicated.

It's really easy to mess up and merge the wrong PR first, and it's a pain in the ass to rebase all the way back up if you make a change in Request 1. I'd love to know how other people are handling this.