Microsoft has released the first Windows 10 build in the Redstone 4 branch to a subset of Fast Ring Insiders.
Windows 10 Build 16353, which Microsoft is rolling out on Aug. 31, is the first test build in the RS-PRERELEASE branch, which is the Redstone 4 (RS4) development branch. RS4 is the version of Windows 10 that will follow the Fall Creators Update and is targeted to be available around March 2018.
Fast Ring Insider testers who are not in the Skip Ahead ring already will not get today's test build. They will continue to get Redstone 3/Windows 10 Fall Creators Update test builds until Microsoft deems that version of Windows as ready to roll out to mainstream users. After that point, Microsoft will transition all Windows 10 Fast Ring Insiders to RS4 test builds. (Those in the Skip Ahead ring also are getting inbox app updates from the Windows Store, while those in the regular Fast Ring are not.)
Microsoft officials have not yet shared the planned name for the next version of Windows 10 client. They also have not yet detailed which features the March 2018 version of Windows 10 will include, though it's likely that it could include some of the features Microsoft postponed from arriving in the Creators Update, such as cloud clipboard, Timeline and more support for Pick Up Where You Left Off.