I read through the changes and have to say that I am really interested in some of the improvements and am looking forward to test them out.
The improved grid system. In some cases, I faced some difficulties with the old grid system, so I hope that the new one will wipe these out.
The improved tooltip placement. Again, I faced problems with the old tooltips. Sometimes they were hidden behind other divs, were miss-positioned, or simply out of the window.
Flexbox support. Honestly, who doesn't want to flex their muscle?... or boxes?
But there are some points I don't agree with (not only bootstrap, but with all design guidelines and frameworks).
I saw the new templates, which bootstrap now sells. (They sell it, or did I saw something wrong?) I saw many homepages designed with bootstrap. And what I don't like is... many people just stop designing once they pick a template. They use 1:1 what they get delivered from these frameworks.
It's like "the best resume on the web" which was posted on devmag. It was good, but not the best, because it was simply using the material guidelines 1:1.
So, I hope that even with the new, better, more fancier and maybe awesomer bootstrap 4.0, designer will start to optimize and individualize their design, and not just think "oh, hey, bootstrap does this right, we use it!".