1) Not sure what you mean? 4.1 runtimes have some differences to add sequences and other things, but otherwise work very similar to 4.0. 4.0 projects will work the same in 4.1, if that's what you are asking. We haven't made changes to the behavior of existing features.
2) Estimates are never a good idea with software. We would like to finish quality of life improvements and add physics in 4.1. Pushing physics to 4.2 is an option, but that would be a bummer.
To use 4.1 while in beta you likely need the runtimes to be updated. We generally update them quite rapidly, with spine-libgdx being done first and then spine-unity being done soon after. Then usually spine-ts is next, then the rest.
@stikkanimate, BTW you once asked about weight transfer from a bone to another bone. You can now do that in 4.1.16-beta, a little explanation here:
4.1.16-beta allows locking bone weights. You can lock all bones except for 2, then remove or add to one of the unlocked bones to transfer weight from one bone to the other. This covers the desired use case and more.
Note that selected bones are always affected, even if locked. So you can lock all bones, unlock one, then select another and adjust it.
To quickly lock/unlock many bones, press an hold on one of the lock buttons (the colored squares), then drag to other lock buttons.