True, but the build 16299.15 that was released was compiled at the end of September, hence it was still officially '1709'.
17134 wasn't compiled until April so it that were to be the release build and MS want to stay consistent in their naming policy, would have to be called '1804'.
I got a copy of the RS4 17133.1 build using Media Creation Tool (i downloaded a file named Products_RS4_03_30_2018.xml and created a products.cab from it otherwise it still gets the RS3 build..). I then burnt a a DVD from the ISO file generated from...
UPDATE 4/20: Windows 10 Insider Preview Fast/Slow/RP Build 17134 - April 20 Insider - Windows 10 Forums
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Source: Announcing Windows 10 Insider Preview Build 17133 for Fast - Windows Experience Blog