Build 4001.main.021204-1515 is the first known Longhorn Milestone 4 build. The build itself remains unleaked, however screenshots have been shown by Dr. Nicholas Rush on Yet Another Longhorn Board and by bashar2000 on BetaArchive [ref].
Taskbar/Sidebar explorer glitches can be seen on last two screenshots, looking similar to 37xx builds glitches.
This is also one of the earliest builds to include the WIM-based Setup routine, basically the first radical departure from the old Windows NT 5 code. 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 almost immediately into the graphical user interface itself.
This was done not only to drastically simplify the Setup process itself, but also to acheive Microsoft's goal of installing Windows in about fifteen minutes, or at least to speed up Windows Setup to some extent.
Gallery
Autorun
Setup from XP
Setup from XP
Setup from XP
Setup from XP
Setup from XP
Setup stage 2
Setup stage 2
Setup stage 2
OOBE
OOBE
OOBE
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