It's currently in beta. According to the developers there will be no more breaking changes, so that's something. It's also the default download version on getbootstrap.com now. For early adopters I think it's a good pick. If you need more stability, Bootstrap 3 will be maintained for some time as well.
Bootstrap 4 has some really good new features so I use it for my own projects.
Bootstrap 4 has some really good new features so I use it for my own projects.