Microsoft Knowledge Base Article 257754 contains the following summary:
Removing Windows 2000 and installing Microsoft Windows NT 4.0 is a manual process; there is no uninstall process available.
This article describes preventative steps that you should take to preserve data before you attempt to remove Windows 2000
and install Windows NT 4.0 on your computer.
NOTE: This article does not describe how to install Windows NT 4.0. For information about how to install Windows NT 4.0, see the product manuals and the Microsoft Knowledge Base.