Yes, wow, a million times yes. I can't imagine starting a project from scratch, I've been using tweaked parts of BS4 in production and I can't imagine going back to writing my own media-query mixins, modal components, etc. Even in alpha it's rock-solid and HUGE time-saver.

I find it to be beyond compare, indeed many other frameworks will copy its component offerings and naming conventions.

Compared to 3, BS4 is really well-refined and solves a lot of issues. That + the flexbox enhancements make it a joy to use, personally.