I made two additions to the BrushesAndStuff toolbar and clicked OK. They appeared and worked fine. The next tie I open Krita they didn't show. I checked the Configure Toolbars dialog and they were in the correct place on the Current actions. I removed them and added them back. Everything was fine until the next time I opened Krita and they were gone. The options were for Brush Smoothing Stabilizer and Brush Smoothing Basic.

Anyone had a similar experience modifying the toolbar in 4.0?

Thanks

I've just tried this in the 4.0.0 and 4.1.0 nightly build appimages and I see exactly the same result as you do. It's the same for all the Brush Stabiliser/Smoothing tools but the other tools I tried did work and were there next time I started Krita. I didn't try many, just a few.

It seems that you need to raise a bug report which is always fun: https://bugs.kde.org/

The actions that are part of a tool cannot be restored to the toolbar, because the toolbar is created before the tool. This is a very long-standing problem that we just have not been able to solve yet. There must be a report for it in bugzilla already but I cannot find it right away.

This looks like the one, raised in 2016: https://bugs.kde.org/show_bug.cgi?id=365222