Today, Microsoft released the first Windows Server Insider Preview since the last feature update was made available. That means that it's from the Redstone 4 development branch, just like the current PC builds. And just like PCs in the Fast ring, the build number is 17035.
Version 1709 included features like the Windows Subsystem for Linux, Linux containers, Delivery Optimization, and more. For now though, there isn't a whole lot that's new in Redstone 4, as today's build includes the following features:
Storage Spaces Direct (S2D) is enabled. Storage Spaces Direct is the foundation of our hyper-converged solution and we’re continuing to evolve it. In this preview build we not only brought it back, but we’re adding some new and important updates to it – such as support for Data Deduplication, a commonly requested feature for Storage Spaces Direct and ReFS. Starting with this build, Data Deduplication will reduce the data footprint by up to 50%.
Developers can now use localhost or loopback (127.0.0.1) to access services running in containers on the host
There are also some known issues to be aware of:
[NEW] The base filtering engine (BFE) service may fail to start, which prevents the Windows Defender firewall (MpsSvc service) from starting.
A system crash may occur because of a kernel security check failure (bug check 0x139) from a corrupt list entry in a deferred free pool.
An application may crash because of a heap entry corruption error (0xC0000374) in the health attestation configuration service provider (CSP) library (Hascsp.dll).\
Testing of the Windows core may fail because of a timeout while attempting to load the test libraries.
Unlike the version of the OS for PCs, there aren't different rings for Server that you can use to get builds at different rates. You can download the images for build 17035 here.
As for when this will be released to the public, the timeline should be the same as it is for the rest of the Redstone 4 branch: sometime in the spring. Between now and then, you should expect to hear a lot more about what will be included in the feature update.