Dona Sarkar (head of Windows Insiders) wrote in a blog post yesterday that Microsoft is releasing two Windows 10 Builds for Fast Ring and Skip Ahead.
Skip Ahead is to SKIP current Fast Ring Builds (for RS4) and start receiving builds for the next version of Windows 10, known as RS5 (Redstone 5).
Right now our focus is on stabilization for RS4. As part of the stabilization process, we have “forked” RS4 into its own branch called “RS4_RELEASE” just like we did with RS2 and RS3. Going forward – RS4 builds will come from the RS4_RELEASE branch. This also means that we intend to release new RS4 builds to Insiders more quickly to both the Fast and Slow rings as these builds we include mostly bug fixes.
Currently Build 17101 for Fast Ring is released which now is more focused on stabilization the system then actually adding new features. RS4 is actually feature complete and is just going through bug fixes and any issues that needs to be resolved.
RS4 is expected somewhere in March/April this year.
Some of the improvements in Build RS4 are:
Emoji Design Updates
Windows App Permissions
UWP is getting more control over full file system. “Particular UWP apps will be granted permission to have broad file system access. This capability will be granted on a per app basis by Microsoft. If a UWP app has the broad file system access restricted capability, you will receive a consent dialog prompting you to accept or deny the request.“
For a complete list of improvements and fixes, head to the blog (link above).