I'm excited to see it was finally released, and I think the main reason of course for not pushing v4 was it wasn't stable yet.
I'm really not sure on the ETA for US5, I've only really heavily integrated myself in US4 and I know there was a particular developer with high plans for US4. @mudmin could chime in on this. Are you in our Discord server? I'd love for you to join us!
If you put the <div class="panel"></div> it doesn't convert indeed. The list next to it states .panel will become .card though
I'm already using Bootstrap 4 on my UserSpice's theme. I think it would be great if it be included by default in core.
The issue is and we've discussed this before-it will break projects. Some users have their site completely decked out with Bootstrap 4 and several classes changed names which will break it. When we make a massive change like this, we run into problems with users sticking back on an older, and what will turn into unsecure version of UserSpice because they don't have the time to make the changes to migrate to BS4.