I also like Dropbox i.e. to copy my code there as backups. I wouldn't suggest working directly with your projects in a Dropbox folder though - it is prone to weird errors which are difficult to track if you do this.
That said, I think the best solution would be an integration with GitHub/GitLab or other similar repository services directly from the IDE. In this way, there is a backup but one can also revert to previous code. There are so many solutions these days which are cloud-based and I think B4X would also benefit from this. It would also be a great way to share code and collaborate with other B4X users.