It’s about time. I started using BS4.0 when it was in beta and never used 3 since then. However, I am now venturing into CSS Grid–designing a theme right now with CSS Grid and Flexbox (no Bootstrap). One of the big issues with Bootstrap is RTL support; lack of. Then, of course, there’s the bloated BS framework. I eventually took snippets of only what I was using in a theme and dumped the rest (usually 90% of it).