Good question! Honestly, we coded things so if you use the Redux API they should be fully interchangeable. There are just more nice features in Redux 4. If I were you I’d build with Redux 4, and then test with Redux 3. We have more than a few developers embedding a version of Redux 4 in their already released products. And I could always use someone else looking for bugs in Redux 4!
As far as a release date, since we have the new site out that’s my next focus. We’ve been working hard to make sure nothing breaks!
And on a side note, you might want to consider one of our access passes: https://redux.io/pricing as it will be the only way going forward that you will get access to certain very useful tools. ??
If you want to chat more, send me an email to [email protected] or join our slack channel at https://slack.redux.io/ (yes, I know I need to add an SSL cert).