one of the things I love about bootstrap is being able to google "bootstrap <some component>" and get something that works and looks nice.
will these third party components need to be re-written and support multiple bootstrap versions?
I can't speak to components, but for themes there's already a migration guide, and I'd expect to see tools that help with the conversion since they're conceptually similar.
https://v4-alpha.getbootstrap.com/migration/
> will these third party components need to be re-written and support multiple bootstrap versions?
They'll need to be updated, but it's a one way trip. It wouldn't surprise me to see popular themes maintain Bootstrap 3 and Bootstrap 4 versions in parallel for a while.
I recall that happened with the Bootstrap 2 and 3 transition with some components maintaining versions for both for a while.
The jump between 3 to 4 isn't as great as the jump from 2 to 3. With the jump from 2 to 3, many items never made the transition. I don't think that will be the case this time around.