More than 20 supercars, including the £1m Pagani Zonda and Ferrari Enzo, are being unveiled tomorrow (13 October) in a car park in Shoreditch, east London.
Passers-by will be able to play the game on a number of Xbox 360 consoles at the car park, in order to take the cars on a "virtual test drive".
Television, digital, outdoor and print activity created by McCann Erickson is also running to support the launch of Forza Motorsport 4, which is released on Friday (14 October).
Microsoft-owned Xbox is also hosting a 7-day event in the Westfield White City shopping centre, organised by UM London and featuring a team of mechanics building a replica GT40 supercar.
The event is being streamed live via a microsite, which is being delivered in partnership with motoring title PistonHeads.
Microsoft's internet portal MSN is also streaming content via its regular 'Car of the month feature' and a time-lapse edit of the entire build will be seeded on MSN and sites handled by Say Media, Collective, Smartclip, EVO, IGN, Future and the Xbox Live platform.
Viewers of the live stream are being asked to suggest ways in which the car can be customised with some of the suggestions being implemented by the mechanics.
Anyone who makes a customisation suggestion will stand a chance to win the car, once it is complete.
Further marketing activity will include a front cover takeover of the Evo motoring magazine, which will feature an image of one of the cars that features from Forza Motorsport 4.
Evo will also push the game via its iPad app and social media channels, including Facebook and Twitter.
In addition, Microsoft has also partnered with Blippar to create an augmented reality app that will allow readers to scan ads with their phones in order to view a video trailer of the game.
Follow Matthew Chapman at @mattchapmanUK