Forza Horizon is more geared towards street racing, whereas Forza 4 is more geared towards actual racing, the cars which can be driven in this game you may see on actual streets. So in essence the two games are modeled quite different.
Super Gamer Dude
In a gaming world full of spin-offs and reboots that end up disappointing the people who made the series successful, Forza Horizon attracted a massive amount of negative attention when Microsoft first showed it off as part of their Xbox 360 lineup at E3. Hardcore racing simulation fans worried that this would be the new direction of Forza, giving it a much more arcade racer-like feel.
To be quite honest, those naysayers were right. Forza Horizon throws much of the simulation aspects out the window, particularly in regards to precision car tuning. However, that does not make Forza Horizon a bad game. In fact, Forza Horizon is one of the best arcade racers in recent memory.
The basic story concept (I didn't realize racing games needed one either), involves a sort of burning man festival equivalent to racing inasmuch that you enter and fight your way through the ranks. As you explore the open world, driving from event to event, everything you do counts towards your popularity level, making the game's end goal two fold. You must not only win the festival by going through all the different colors of armbands, but also become the most popular racer at the festival.
The game's handling model, while not as advanced as that of Forza, feels quite a bit like one of the DiRT or Project Gotham Racing games. While the combination might seem strange at first, it begins to make sense when you recognize the pedigree of the team that developed the game. Playground games is made up of former members of both Bizarre Creations and Codemasters, and the two design philosophies blended together manage to straddle the line between simulation and arcade racing.
While the driving mechanics are often the most important part of any racing game, Forza Horizon has something that is at least of equal importance, its world. The festival itself is largely irrelevant to the world, and it only serves as a simple framing device, but the environment it takes place in is absolutely stunning. This fictional location, somewhere in Colorado, creates opportunities for incredible vistas all the time, and you will often find yourself distracted by some absolutely incredible scenery as you make your way down its long winding roads.
This scenery, combined with the fact that you are always earning popularity, make driving between events a treat instead of the chore it usually is in an open world game. Any time you pull up behind another car, you can challenge them to a race, or you can just drift around every corner and try to zoom past every single speed camera as fast as possible.
With Forza Horizon we have yet another reboot of a popular franchise that ends up creating something completely different. Whether or not its better than old Forza is for you to decide, but as far as I'm concerned, Horizon is a fantastic open world racing game whether you've enjoyed Forza's style of racing game in the past or hated it.