As the Windows 10 Fall Creators Update is now fully baked, Fast Ring Insiders received their first Redstone 4 build today, along with “Skip Ahead” Insiders. In the blog post announcing the news, Head of the Windows Insider Program Dona Sarkar explained that starting today, Fast Ring and Skip Ahead Insiders will receive future RS4 builds at the same time:
Going forward, we plan to release new builds for RS4 from the RS_PRERELEASE branch to Insiders in the Fast ring and those who opted in to Skip Ahead at the same time. Because we are just beginning development for RS4, Insiders shouldn’t expect to see a lot of big changes or new features just yet. But teams are just now beginning to check in new code!
If Fast Ring Insiders will now receive the same RS4 builds as Skip Ahead Insiders, it's likely that the latter ones will continue to receive updates for stock Windows 10 apps before everyone else.