Today, Microsoft announced that Windows 10 SDK Preview build 17046 is available, although the download has been listed for a couple of days now. It's the first new SDK since build 17040 was released 10 days ago, so these are coming in a lot more frequently than they used to.
Indeed, SDK Preview builds used to align with the Slow ring, which receives Windows 10 builds monthly. Now, they seem to coincide with the latest Fast ring build. That means that this is the SDK for Redstone 4, or the next feature update for the OS.
There doesn't seem to be any major changes in this SDK either. The announcement lists C++/WinRT under "what's new", and new MIDL key words under "breaking changes", but these were in the 17040 announcement, as well as in previous builds.
The known issues are the same as well. All tests that are run with the Windows App Certification Kit won't work, and installation is unsupported on non-Insider Preview machines.
To download Windows 10 SDK Insider Preview build 17046, you can find it here.