Nukkit for Minecraft 1.4.0 can now be found on our Jenkins CI starting from build #165.
Edit: 1.4.1 and 1.4.2 are just client fixes! Players will still be able to join.
This comes with no support for the new blocks added in Update Aquatic since it will require some major block and chunk rewriting or a temporary hack to convert legacy block IDs to new run-time IDs for the paletted chunks.
If you find any bugs on the latest builds, please report them to the GitHub issue tracker unless they have already been reported.