Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
Landing UI Kit - Handcrafted UI components for landing pages
Bootstrap Zero - Open-source, free Bootstrap templates collection.
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
HackerThemes - Bootstrap 4 themes and tools for web developers