http://wccftech.com/windows-10-redstone-4-preview/

Microsoft is releasing its first Windows 10 Redstone 4 build to the Fast Ring (Skip Ahead) insiders ahead of the public release of Windows 10 Fall Creators Update. The Windows 10 Insider Preview build 16353 is only available to those who had opted in to the Skip Ahead branch. Microsoft had announced this new subset of Fast Ring back in July, but they are only now receiving the first RS4 build. While the Windows maker is no longer accepting any requests to Skip Ahead, those who did opt in and were accepted will continue to receive builds from the RS_PRERELEASE branch which has now transitioned to RS4. Redstone 4 is expected to be released in Spring 2018.

Until now, Skip Ahead was receiving the same builds as the Fast Ring insiders. But, from today on the builds will no longer be same as the Fast Ring is moving to the stable Windows 10 Fall Creators Update builds and Skip Ahead is going to get early peek into the Windows 10 Redstone 4. Microsoft apparently realizes how confusing the entire testing community is now getting with multiple rings and now the subsets of rings. “We recognize this might seem a bit confusing to some Insiders but with Skip Ahead, we are able to test our ability to release builds from two different branches at the same time,” the company tried to clarify.