Today is Patch Tuesday, and while many thought that that meant that Microsoft might release the Windows 10 spring update today, that's not what happened. What is happening, however, is that Microsoft is releasing the first Redstone 4 cumulative update to Insiders on the Fast, Slow, and Release Preview rings.
Redstone 4, also known as version 1803, still doesn't have an official name. Presumably, this will be announced soon. One thing that Microsoft always does before public releasing an update is test out a cumulative update, since this is how the build will be maintained. This is that test.
Build 17133.73 includes the following fixes:
Addresses a PDF security issue in Microsoft Edge.
Addresses an issue that, in some instances, prevents Internet Explorer from identifying custom controls.
Security updates to Internet Explorer, Microsoft Edge, Microsoft scripting engine, Windows kernel, Microsoft graphics component, Windows Server, Windows cryptography, and Windows datacenter networking.
Microsoft didn't note any known issues in today's cumulative update, and it's certainly not a long list of fixes. It's an important milestone though, because assuming that users don't have issues with installing this update, then Windows 10 version 1803 should be ready to roll out to the public.
At this point, it's safe to assume that non-Insiders will see Windows 10 version 1803 within days, or possibly next Tuesday, as not to conflict with Patch Tuesday.