You can download old builds via:

The release is a specialized ISO image of the Windows 10 Pro operating system, tailored for users seeking a pre-configured, updated environment from mid-2018. Based on the Redstone 4 (RS4) development cycle, this build corresponds to the April 2018 Update (Version 1803), aimed at enhancing user experience with improved security and productivity tools.

Build 17134 was then quickly rolled out to Insiders on April 16, 2018, and after being vetted, it was confirmed as the new RTM build. This final build was officially released to the public as the Windows 10 April 2018 Update on April 30, 2018, with a phased rollout beginning on May 8.

: This stands for "Redstone 4" , the internal development codename used by Microsoft for a specific wave of updates. The Redstone series was Microsoft's approach to delivering major feature updates to Windows 10 on a semi-annual cadence. Redstone 4 was the fourth such update, following Redstone 3 (the Fall Creators Update). It culminated in the public release known as the Windows 10 April 2018 Update .