They go together; Bootstrap 4 will have an option to use flexboxes to implement its grid system. It's an option and not the default because there are a lot of

They go together; Bootstrap 4 will have an option to use flexboxes to implement its grid system. It's an option and not the default because there are a lot of browsers out there that don't support flexboxes yet; give it a few years and this will be a much more practical choice.