With Microsoft clarifying that with Windows 10 as Service we will see a new feature update every six month, it has become easy to guess version number for upcoming Windows 10 major updates. We know that upcoming Fall Creators update aka Redstone 3 will be knows as Windows 10 Version 1709.
Now, an official documentation shows that Microsoft already refers the next update aka Redstone 4 as Version 1803. This confirms that Microsoft intends to release this update in March 2018. Not only this even update next to Version 1803 is mentioned as Version 1809, which puts its intended release in September 2018.
The official explanation lays bare the name and release date for Windows 10 Version 1803. As mentioned in the document,
In this new model, Windows Server releases are identified by the year and month of release: for example, in 2017, a release in the 9th month (September) would be identified as version 1709. Windows Server will release semi-annually in fall and spring. Another release in March 2018 would be version 1803. The support lifecycle for each release is 18 months.