Yes, Nebula 4 is being actively developed - you can already experience many improvements related to engine itself (zero-latency etc.). We are seeing major progress (more to come), so this can be considered as a part of the new major version.
The other side, there is a development branch related to a new user front-end. The foundation is currently being ported to JUCE framework.
When both development branches will merge into one product, we will have real Nebula 4 on the table