Based on the v4 development cycle, I estimate that a stable release 5 will be around Oct 2017 (actual production release, not a release candidate or beta). This is based on a 9 month period from when a feature freeze is declared (v5 is not frozen yet, but promised soon).
There are some features from the v5 roadmap that are being bumped to v6. I expect v6 to be sometime in 2019.