Windows Longhorn build 4001

Windows "Longhorn" build 4001 (with a build tag of build 4001.main.021204-1515) is the first Milestone 4 build of Windows "Longhorn". It was leaked to BetaArchive in January 2013. Prior to its leak, screenshots of this build were shown by Dr. Nicholas Rush at YouArePwned's "Yet Another Longhorn Board", and by user bashar2000 on the BetaArchive forums.

This build is among the earliest builds to use a new WIM-based setup routine, one of the first radical departures from the previous Windows NT codebase. The WIM-based setup routine works by having all of the setup files stored within what is essentially a single disk image, INSTALL.WIM, and, when booted directly from the media itself, loads the WinPE (Windows Preinstallation Environment) and boots into the graphical user interface itself. Microsoft stated that their goal with this routine was to allow installations of Windows to finish in as little as 15 minutes.

Taskbar and Windows Sidebar bugs visually similar to those in 37xx builds are present in this build.