Today, Microsoft released Windows 10 for PCs Insider Preview build 17063, the first new build in almost a month, containing a ton of new features. Alongside of it, the firm also released a new SDK Insider Preview build, 17061.
The main changes that you'll find in the Redstone 4 SDK is that C++/WinRT are now included in the Windows SDK, and there are new MIDL keywords. Neither of these features are exclusive to build 17061; they were in earlier SDK Previews.
There are also some known issues to be aware of. As has been the case for a few builds now, you'll need to be on a Windows 10 Insider Preview build in order to install it. It will also fail if you try to deploy your app to a mobile device, and the XAML Designer will fail to load. Obviously, these issues are significant, so you won't want to use this in a production environment.
If you're a developer, you'll want to check out the full list of API updates and additions, which you can find here.