First, great job on v4!
Second, I want to bring up the issue of category links in cut & pasted matrices. This is something that has been a source of pain for a while in v3, but I thought someone might have fixed it going into v4. I just downloaded the beta, and it appears that no one has.
To replicate: create matrices, link their categories, and then cut and paste one or more of them in the Model Browser. You’ll see that the categories in the cut and pasted matrices lose their links to any matrices not involved in the cut and paste.
In large models, this problem can create all sorts of hell, because if one cuts and pastes many linked matrices at once, they will keep their links to each other but lose their links to the remaining matrices in the model. They will therefore keep the little arrow on their category names, making it extremely hard to figure out that broken links are the source of the problem.