Bootstrap V4Browser Info Environment
I know what you mean about starting with the newest version; however, we have no immediate plans to update our themes to Bootstrap 4. I would suggest having a look on the marketplace to see if there is a Bootstrap 4 theme.
There is a starter theme on Github here for people to use as a starting point to building their own theme.
Might be worth asking on the concrete5 forum if anyone knows which themes use Bootstrap 4 - I'm sorry, but I don't know which ones do. Alternatively, you could post a job on the jobs board for someone to build you a custom Bootstrap 4 theme.