I’ll do this stackoverflow style for everyone who runs into the same problem:
When I switched to ST4 (which is an amazing upgrade is works so nice!) I noticed that Goto Definition is not working for all symbols, I assumed an indexing problem.
Not so! It’s a feature that has already bitten me in VSCode.
"index_exclude_gitignore": true,
This is a new setting, that by default ignores everything in your gitignore. That includes e.g. installed libraries that you do want to navigate through. Just set this to false in your user config and you’re good to go!