According to a Microsoft statement received and published by several gaming news sites, the video "was not intended for public distribution and may not accurately reflect features and functionality that ship in Forza Motorsport 4." The company's concern may lie in promising features that have yet to be finalized, such as 16-player online races, the contribution of over 80 automobile manufacturers and the introduction of social car clubs.
(One promoted feature we can rule out with ease, however, is the career mode that has you "literally" traveling the globe. Forza 4 isn't likely to include a bundle of plane tickets -- if it did, you'd have a cheap opportunity to swing by this year's semantics nazi convention.)
Considering the glossy, flattering nature of the video, and the presence of already-announced additions like Kinect head tracking, it's possible that April Fools' Day marked the start of a viral marketing campaign for Forza 4. Let's see how long this version of the video stays up, and whether Microsoft comes knocking on our door. Not literally.