While Microsoft has never made a public announcement about the release of Windows 10 1803/Redstone 4/Spring Creators Update/??, tech pundits with inside the company connections were expecting an announcement and rollout beginning on April 10th, last week. Indeed, there were a number of indications that we were close: a few last minute changes via Fast Ring updates, then a release of build 17133 to both the Slow and Release Preview rings. Microsoft promised those RP updates would roll out over the weekend, but something happened.
After a Slow Ring update to 17133 happened on March 30th, Microsoft released that build to the Release Preview Ring on April 5th, announcing it with an update to the original 17133 Fast Ring announcement:
UPDATE 4/5: We have released Windows 10 Insider Preview Build 17133 (RS4) to Windows Insiders in the Release Preview ring. Not all Insiders in the Release Preview ring will be immediately targeted to receive this flight.
We are targeting full availability to Release Preview Monday 4/9. However, if you’d like to install immediately, a manual check for updates under Settings > Update & Security > Windows Update will present the build.
(ed. note: the crossed out notation was added with the 4/9 update below, the original effort was to get 17133 out to all Release Preview Insiders by last Monday)
Then on April 9th, the post was updated again:
UPDATE 4/9: We are continuing to test our targeting and as a result, not all Windows Insiders in the Release Preview ring will see Build 17133 offered at this time. We’ll update this post when full availability is reached.
While there was some speculation that the April 10th rollout was delayed to a last minute change to the marketing name for Redstone 4 (indications had pegged the name as "Spring Creators Update," but perhaps due to the recent upheaval to the Windows team, that name has reportedly fallen out of favor), Microsoft watchers seem to agree that this delay is based on last minute bugs found possibly in the early release of the Release Preview build of 17133.
Microsoft has since released a new build for Fast Ring Insiders, version 17133.73, with a few bug fixes, but internally the company has moved on to at least build 17134, with that build number to possibly go higher before Redstone 4 is finally rollout out to the masses.
So in a nutshell: Microsoft was ready to deploy a final version of Windows 10 1803 (or 1804, really: the 4th month of 2018), rolled out a build to Release Preview Insiders, hit a bug, and have pulled back to fix it. This is precisely what the Windows Insider system is supposed to be for, setting aside all the #Insiders4Good, A/B testing, and marketing fluff nonsense that has bloated the program.
So if you're upset you're not getting a finalized version of Redstone 4, don't be, and be thankful that what appear to be some potentially nasty bugs have been caught. If you're waiting for the Next Big Thing, that's what the Skip Ahead program is for, and Insiders no longer need to wait until this update is out the door before getting new bits. And if you believe every pundit that "scoops" a final date that then gets canonized by every tech website in the world, then you don't understand that it's not that set in stone internally, and we shouldn't expect it to be set in stone out here either.
Relax, wait for a new build (or two or three), and relish in the fact that as Insiders, we've done our job.