CodeIgniter 4.0.0-beta.3 released

If I start doing something on beta.3 today, how seamlessly will I be able to upgrade to 4.0.0?

From the v4 documentation

CodeIgniter 4 is a rewrite of the framework and is not backwards compatible. It is more appropriate to think of converting your app, rather than upgrading it. Once you have done that, upgrading from one version of CodeIgniter 4 to the next will be straightforward.

So, not seamless, but not a complete rewrite either.