Unfortunately, the upgrade to Bootstrap 4 is not planned yet.
The main problem is that they have changed many classes, and the new ones are not retro-compatible with the old ones, which means that applications that used Bootstrap’s UI elements will no longer work after the move to Bootstrap 4.
I have been very disappointed by this framework for this reason. They do not consider retro-compatibility. Even the CSS preprocessor has been changed (from LESS to SASS) and they said that they might use something else for Bootstrap 5. In my opinion, it’s a framework for the “cool” front-end guys that do not care about products that should be maintained over years.
Anyway, I don’t think you absolutely need Bootstrap 4 to create a nice app. Bootstrap 3 is still suitable in my opinion.