To be clear, v4.2 is the stable dependable target that most users should be using as their target. Developers that want to track new development and new features and have more flexibility can move up to v5. Next year at around this time, v6 will become the new stable target. The actual number of breaking changes between v4 and v6 are likely to be quite small -- things that work in v4 should continue to work in v6 and beyond really. So the best advice I can give is: if you are developing modules for developers to use in their applications, you'll want to primarily target v4 for now but keep an eye on where v5 is going and use that as your beta channel for new development going forward.