Bootstrap Magic - Create your Bootstrap 4.0 themes easily with magic
Froala Design Blocks - 170+ free responsive design blocks based on Bootstrap 4
HackerThemes - Bootstrap 4 themes and tools for web developers
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.