Windows Longhorn build 4017 is a leaked build of Windows Longhorn. Grabberslasher, the owner of the UX.Unleaked blog listed this build in a thread on BetaArchive to be released alongside build 4029. This build is very similar to build 4015.main.
This build cannot be clean installed due to the data for most files only being able to be expanded correctly on an upgrade from a previous build like build 4001. Attempting to clean install often leads to the setup hanging on a "Please wait..." screen.