For Bootstrap 4 have look at Vartheme (Bootstrap 4 - SASS)
Vartheme BS3 (Bootstrap 3 LESS) is a base theme for Varbase standard websites.
Based on the Bootstrap theme.
Default LTR Vartheme View
Default RTL Vartheme View
Use With Varbase Distribution:
This theme is best used with Varbase distribution.
Can be installed with any Drupal 9 site. Even if installed with the Minimal or Standard profile.
However, using it with Varbase gives you way much more cool stuff!
Varbase documentation
Check out Varbase documentation for more details.