Microsoft's Windows 10 updates naming format usually follows the sequence of numbers representing year (yy) and the month (mm) of the release - as in 2020's first-half release should translate to 20H1; but that has changed with the latest beta build of the upcoming upgrade.
The next Windows 10 refresh has been tagged with the four-digit label of 2004, which according to Microsoft, the designation is supposed to help avoid confusion with the rather erstwhile Windows server 2003 in admission of the name change in the upgrade's release timeline.
While Microsoft released Windows Server 2003 in April 2003, which was eventually retired in July 2015, and superseded by the Windows Server 2008.
Though, Microsoft's naming of Windows 10 upgrades has been rather irregular in format, like Windows 10 1607, for instance debuted August 2, 2016, and not July as the name indicated. The upgrades Windows 10 1703 and 1803 debuted in April, while the 1903 shipped in May. As such, no guarantee that Windows 10 2004 will eventually go live next April.
It's still unclear when Microsoft will begin the beta testing, but if the company should follow the path of the year, it may likely skip 2020's upgrade for fall (2009 or 20H2) and, just like 2019, Insider participants will get very early versions of the 2021 spring upgrade (2103 or 21H1).
Albeit, Microsoft's code names are rather confusing, and the major-minor model of 2019 would be as a one-off, while those of 2020 would return to normalcy, at least if Microsoft's evolving service policies can be a bit normal.
Microsoft has already released Windows 10 Insider Preview Build 19033 (20H1) for Windows Insiders in both the Fast and Slow ring. But has declined to say which exactly will begin testing for early 2020, since the recently released Windows 10 1909 was a minor upgrade.