This build was given out to attendees of the Professional Developers Conference (PDC) 2003 and is the very first version of Longhorn to be released by Microsoft publicly. Build 4051 was lacked most, if not all, interesting bits shown by Hillel Cooperman using a 4050 compile. As such it lacked Aero Glass and Aurora in the preview pane. Slate was introduced as temporary substitute for the Aero theme. Microsoft representatives commented on this fact that the Aero look was still in development and under heavy security.