It appears that the name of the upcoming feature update for Windows 10 is finally known through the official Edge browser landing page which is shown upon completing the update to build 17134, and that is - as shown above - named as the "April update", I stumbled on it when updating my laptop to build 17134, which was released to the Slow and Release Preview rings on Friday.
An earlier report here on Neowin had suggested the official name would be the "April 2018 update", because a Microsoft employee says it in a video that was discovered by Twitter user WalkingCat earlier this week.
Over on WindowsLatest, Mayank Parmar discovered that the Microsoft Edge browser recently received a server-side update in the Redstone 4 builds, with the browser now displaying the updated welcome page, confirming the "April update" nomenclature. The welcome page promotes features of the April update, including Windows Timeline. You can view it yourself here.
In any case, Microsoft still hasn"t announced the official name in any official blog posts for Redstone 4 build announcements. As a reminder, previous suggestions of the name for this update were: Spring Creators Update, Spring Update, and most recently, the April 2018 Update.
The last time Microsoft used a month name in a feature update for Windows 10 was with the first major feature update for the OS, version 1511, which ended up being called the November update, released in 2015.