The next version of Windows 10 was supposed to have version number "2003" but Microsoft decided to skip this number in favor or "2004". The Register writes Microsoft isn't using 2003 naming for the Windows 10 20H1 Update because it would be too similar to Windows Server 2003.
And because it wouldn't be Windows without a good few ways of referring to it, 20H1 will also now be known as "2004" rather than "2003" as one might have expected, based on the previous numbering convention (1909, 1903, 1809 etc).
The reason, according to the gang, was to "eliminate confusion with any past product names" such as Windows Server 2003. Presumably Microsoft Money 2004 was not deemed risky enough.
It seems like version 2004 of Windows 10 will mainly have "under the hood" fixes, just like 1909 it will be light on new features.