When you preorder the game at selected retailers, you can get a download card for one of those five cars (check here for your country):
Turn 10 are also doing preorder incentives themselves. You can get the Mazda RX-7 Spirit in Forza Motorsport 3 (one of the unicorn carrying over in your Forza Motorsport 4 garage) and two cars in Forza Motorsport 4. Check the procedure to get get those free cars on this page.
American Muscle car pack includes :
1965 Pontiac GTO
1968 Pontiac Firebird
1970 Buick GSX
1970 Dodge Cornet Super Bee
1966 Chevrolet Nova SS
1969 Shelby GT500 428CJ
1971 Plymouth GTX 426 Hemi
1967 Dodge Cornet W023
1968 Dodge Dart HEMI Super Stock
1964 Ford Fairlane Thunderbolt
Launch Bonus car pack includes:
1965 Ford Mustang GT Coupe
2011 Koenigsegg Agera
1997 Lexus SC300
2011 RUF RGT-8
2011 Tesla Roadster Sport
VIP car pack includes :
2011 Bugatti Veyron Super Sport
2011 Ferrari 458 Challenge
2011 Lamborghini Gallardo LP570-4 Superleggera
2010 Noble M600
2011 RUF Rt 12 R
All those car packs will probably be available at a later date on the Xbox Live Marketplace.
To know about all the changes in the gameplay, just go read the "Under the hood Part 2" article on Forzamotorsport.net. Dan Greenawalt goes in details about what has been modified, improved and how Turn 10 changed their whole methodology regarding tires physics.
The most up to date and complete car list can be found on BADNED's Forzacentral. Check it here.
Forza Motorsport 4 can read your Forza Motorsport 3 save.
If you have leveled in Forza Motorsport 3, you'll get those cars :
And if you have one or more of these car in your FM3 garage then you'll get them right from the start in Forza Motorsport 4 :
Both lists are cumulative. The secret car is the
Delorean DMC-12
You can also import your vinyl groups from Forza Motorsport 3.
Official screenshots, leaked screenshots and captures of offscreen videos.
Destructoid: 10
Eurogamer Italy: 10
Eurogamer Spain: 10
Digital Spy: 5/5
Gameblog.fr: 5/5
360 Magazine: 5/5
Xboxygen 5/5
NZ Gamer: 9.6
IGN: 9.5
Nowgamer: 9.5
MSXbox World: 9.5
Strategy Informer: 9.5
Games On Net: 4.5/5
Joystiq: 4.5/5
Telegraph UK: 4.5/5
MondoXbox: 9.4
videogameszone.de: 93%
gamepro.de: 93%
3D Juegos: 93
Gameinformer: 9.25
Xbox360achievements.org:91
JeuxVideo.fr: 9
Eurogamer: 9
Videogamer: 9
Blistered Thumbs: 9
Gamereactor Denmark: 9
Games Radar: 9
Metro UK: 9
Gamer365.hu: 9
incgamers.com: 9
CVG: 8.7
Gamespot: 8.5 (video review)
Gameswelt.de: 84%
Pocket-lint.com: 4/5
PressFire.no 5/6
Gamer.no 8/10
nofi's awesome write-up for thesixthaxis.com
golem.de review (no score)
Forza 4 Official Livery | Photos Thread
November Speed Pack - Available
2012 Aston Martin Virage
2011 #2 Audi Sport Team Joest R15++ TDI
2011 Audi RS3 Sportback
1957 Chevrolet Bel Air
2012 Dodge Charger SRT8
2011 Ferrari #62 Risi Competizione F458 Italia
2013 Ford Focus ST
1992 GMC Typhoon
2012 Lamborghini Aventador LP700-4
2011 Volkswagen Scirocco R
December IGN Pack - Available
1986 Alfa Romeo Spider Quadrifoglio Verde
1967 Chevrolet Chevelle SS-396
1977 Ford Escort RS1800
2011 Ford F-150 SVT Raptor
1957 Maserati 300 S
2010 Maserati GranTurismo MC GT4
2011 Mazda RX-8 R3
1988 Pontiac Fiero GT
2011 Suzuki #1 Monster Sport SX4 Hill Climb Special
1981 Volkswagen Scirocco S
January Jalopnik Pack - Available
1970 Alfa Romeo Montreal
2011 Audi RS5
1996 Chevrolet Impala SS
2000 Fiat Coupe 20V Turbo
1973 Ford Pinto
2011 Honda #33 Level 5 Motorsports Lola B11/83 HPD
2012 Honda Civic Si Coupe
2011 Lamborghini #08 West Racing Team Yokohama Gallardo LP560 GT2
2012 Pagani Huayra
Hyundai Veloster Bonus Pack (FREE) - Available
2013 Hyundai Veloster Turbo (Autovista)
2011 Hyundai #67 Rhys Millen Racing Veloster
February ALMS Pack - Available
1992 Alfa Romeo 155 Q4
2011 Audi #2 Audi Sport Team Joest R18 TDI
1986 Audi #2 Audi Sport quattro S1
2011 Bentley Platinum Motorsports Continental GT
2010 Dodge Quinton "Rampage" Jackson Challenger SRT8
1995 Ford Mustang Cobra R
2011 Holden HSV GTS
1991 Mazda #55 Team Mazdaspeed 787B
2011 Panoz #050 Panoz Racing Abruzzi
2011 Volkswagen Polo GTI
March Pirelli Car Pack - Available
1986 Alfa Romeo GTV-6
2011 Aston V12 Zagato Villa d'Este
1995 BMW 850CSi
1969 Chevrolet Corvair Monza
1969 Chevrolet Nova SS 396
1972 #2 Ferrari Automobili 312PB
1966 Ford Country Squire
2010 Renault Clio RS
2010 Ultima GTR
2011 Wiesmann GT MF5
April Alpinestars Car Pack - Available
2011 Aston Martin #009 Aston Martin Racing AMR-One
1995 BMW M5 E34
1959 Cadillac Eldorado Biarritz Convertible
2012 Jaguar XKR-S
2011 Maserati Quattroporte Sport GT-S
1985 Mazda RX-7 GSL-SE
2011 Mercedes-Benz #35 Black Falcon SLS AMG GT3
1972 Mercedes-Benz 300 SEL 6.3
2012 Scion tC
1997 Volvo 850 R
May TopGear Pack - Available
1977 AMC Pacer X
1965 Austin-Healey 3000 MkIII
2012 Chrysler 300 SRT8
2011 Ford Transit SSV
2012 Hennessey Venom GT
2012 Jeep Wrangler Rubicon
1966 Lotus Cortina
1992 Mitsubishi Galant VR-4
2012 Smart ForTwo
1990 Subaru Legacy RS
Porsche Expansion Pack - Available
2011 #045 Flying Lizard Motorsports 911 GT3-RSR (997/2)
2008 #2 Gruppe Orange Racing 911 GT3 Cup (997)
2008 #7 Penske Racing RS Spyder Evo
2008 #16 Dyson Racing Team RS Spyder Evo
1987 #17 Rothmans Racing Porsche AG 962c
2011 #17 Team Falken 911 GT3-RSR (997/2)
2011 #23 Alex Job Racing 911 GT3 Cup (997/2)
1998 #26 Porsche AG 911 GT1-98
2005 #31 Petersen-White Lightning 911 GT3-RSR (996)
2011 #54 Black Swan Racing 911 GT3 Cup (997/2)
2005 #66 AXA Financial/Jon Groom Racing 911 GT3 Cup (996)
2007 #80 Flying Lizard Motorsports 911 GT3-RSR (997)
1955 550 Spyder
1973 911 Carrera RS 2.7 Touring
1995 911 GT2 (993)
2012 911 GT2 RS (997/2)
2004 911 GT3 (996)
2007 911 GT3 (997)
2007 911 GT3 RS (997)
2010 911 Sport Classic (997/2)
1982 911 Turbo 3.3 (930)
2007 911 Turbo (997)
1970 914/6
1989 944 Turbo
1987 959
2010 Boxster S
2003 Carrera GT
2012 Cayenne Turbo
2012 Cayman R
2010 Panamera Turbo
The Porsche Expansion Pack also features 20 new career events featuring Porsche cars, 250 gamerscore worth of achievements and Porsche rivals event.
Meguiar's Car Pack - Available
2011 Aston Martin Cygnet
1959 BMW 507
2011 Chevrolet #4 Corvette Racing C6.R GT2
1940 Ford De Luxe Coupe
1997 Maserati Ghibli Cup
2012 Mercedes-Benz C 63 AMG Coupé Black Series Aerodynamic Package
1958 MG MGA Twin-Cam
1987 RUF CTR "Yellowbird"
1992 Toyota Celica GT-FOUR RC (ST185)
1963 Volkswagen 1200 "Beetle"
Thanks to soundscream for his OT title, BADNED for his car list, Phonomezer for the review score list.