17604 is the first Redstone 5 build for Windows Insiders
Skip Ahead Insiders can get the first preview build of Redstone 5
Microsoft kicks off the preview for the Redstone 5 update for Windows 10, expected in Fall 2018. Windows 10 build 17604 is now available to download for Insiders in the Skip Ahead ring, while those on the Fast ring can install build 17101 from the Redstone 4 branch.
As the Redstone 5 branch is in a very early stage of development, builds 17604 and 17101 are almost identical, but Redstone 5 should start picking up new features over the next Insider releases. At the same time, Microsoft should be in the final stage of finalizing Redstone 4 for the official launch in March.
As far as Windows 10 Mobile is concerned, the company has practically stopped the development of the mobile version after Redstone 2 (feature2), but the supported smartphones on version 1709 will be getting cumulative updates with improvements and fixes until late 2019. Microsoft yesterday rolled out the February 2018 update for Windows 10 Mobile.