Window 10 Redstone 4 development is done as Microsoft have pushed build 17133 to the Release Preview ring. Only finished builds get to the Release Preview ring so this will be the build that rolls out to Windows users probably as soon as April 10th (the next Patch Tuesday). The Redstone 4 builds have been stable for some time and I have found 17133 rock solid.
Microsoft say if you are set to receive Release Preview builds you will get the build April 9th, but if you check for updates in Settings/Update & Security/Windows Update you should get the build now. The build is currently installing on my Surface Pro 4 via Windows Update.
Here is my hands on with build 17133, and lookout for my Redstone 4 feature video soon.