Forza 4 has a growing number of downloadable content (DLC). Each of them bearing a new selection of cars, and occasionally a new circuit to race on. Whilst it's not a bad thing having regular content, it is annoying when all that content is, is essentially a skin that goes fast or slow. But i can see the appeal, it does sound like an interesting concept to upgrade a smart car and make it race a Porsche... But i digress.
When i get DLC i want to get something new and exciting, like a new world to explore, Forza can't do that so it tries with new circuit and falls flat on it's face covered in it's own urine. Eventually, every game will get boring, and for me, Forza just does it quickly. The point of releasing DLC is to make you want to play the game more. But it just doesn't do it for me. A large part of myself just can't help but see it as monotonous.
My personal opinion would be to not get the DLC, for reasons stated above. But if you love Forza 4 so much, and need more of it in your life. Then I'm not stopping you.