If you have a Volume Licensing Service Center (VLSC) or Visual Studio subscription, you'll find the new Windows Server build 2004 there.
The Windows Server evaluation ISO is build 1809.
You can also download the Windows Server insider preview builds from here:
Also note that the Windows Server, version 2004 is not Windows Server 2019, they are different releases.
- Windows Server 2019 is a Long-Term Servicing Channel (LTSC) release.
- Windows Server, version 2004 is a Semi-Annual Channel (SAC) release.