Before I put your hopes high and then down like it happened to me, you can’t play the Warthog on Forza 4. Though I’m not a fan of Halo but the design and driving experience of the warthog is one so unique and no one ever came close to creating anything like it.
There has been drivable real-life ones but never featured in another video games. Since Forza and Halo franchises are owned by Microsoft Game Studios, they decided to yet milk the Halo series into another medium. Like I said it won’t be drivable but instead it will be viewable in a showroom on Forza 4 where you get to learn more about the car and admire it’s beauty voiced by Cortana.
Turn 10 studios who created Forza said that we can’t play it in the game because the car is huge and will crush other cars, but I have a suggestion for them, just have a Warthog only race mode and you boost the game sales by a whole lot. Anyway, here is the video of the Warthog in the game.