Disclaimer: I do compare this game to Gran Turismo 5 a lot This is necessary due to the fact that it's the only other racing sim out there.  At no point is it my intention to come off as an XBox fanboy. I do have both systems and own both games.
For years, the best racing simulator money could buy was Gran Turismo on the Playstation.  In 2004, however, Forza Motorsport for the XBox broke onto the scene and upped the ante in a lot of ways, but graphically, Gran Turismo always had it beat... until now.  While I have always preferred Forza to GT, the fourth installment in the series is the first one that allows me to definitively say that the franchise has beaten out its competitor in every way.

Gran Turismo 5 was a huge disappointment for a lot of fans.  After six years of development time and multiple delays, the game failed to deliver on anything new besides a graphics update.  Opponent AI, something that the game hadn't improved on since its original iteration back on the first Playstation console was left unchanged.  Free play mode, a mode that should give you access to all the game's content independently of other modes so that you could experiment with cars and tracks for fun if you wanted to, required things to be unlocked in career mode.  Fans were left wondering what they had waited six years for.  All in all, the game was mostly a letdown and fans of racing games didn't have much to look forward to... until now.

Graphically, Forza 4 is amazing.  Graphics have been the one realm where Turn 10 studios has seemed to lag behind Polyphony digital, not anymore.  In race graphics are staggeringly good looking across all areas of the game.  Cars look realistic and they didn't slack on making the tracks look shoddy earlier.  This is truly a beautiful game that is a sight to see whether you're playing yourself or watching a friend play.  To add to that, Autovista mode offers a selection of cars to explore in meticulous detail down to accurate animations of hydrualic door hinges and engine covers, and incredibly detailed textures down to the leather on the seats and dashboards, and the stitching in the seat belts and embroidery of the logos on the seats.  This mode offers as much information about the cars and you would ever want and even includes commentary on each vehicle by Top Gear's host Jeremy Clarkson, and while he is a little less irreverent here than he is in the show, it's still entertaining.

Notice the realistic detail in the wood grain on the steering wheel, as well as the fact that you can read the writing on the bolts on the steering column and the dashboard.

Lighting, too, has been retooled for this installment of the series.  In particular events the sun will start to set during the race and change the look of the game as you're playing.  This may be a small detail, but is an improvement to Forza 3 where almost all the races tended to occur during broad daylight.  It's not really possible to say that Gran Turismo has the edge in graphics anymore.

Gameplay is top notch as well.  One of the biggest faults about GT5 last year was that it didn't play well with the controller.  The directional pad was too unresponsive and the analog stick was too sensitive, which made making small steering adjustments difficult.  The mapping of the controls was a bit off too, using the face buttons to control gas and braking offered too little sensitivity.  Mapping them to the triggers just felt awkward, which is a fault of the Playstation's controller design more than a problem with the developers.  Ultimately, you had to buy a steering wheel to make the game control the way it should, but that could be expensive with wheels costing up to $300.   

Forza, thankfully, plays well with the hardware you already have.  The analog stick has just the right amount of sensitivity and throttle and braking are controlled with the triggers, allowing you to as much as you need to take corners.  

The game is also compatible with Kinect.  You can do some single player free play races using the Kinect as the controller, and walk around in Autovista mode.  This is kind of fun but mostly a novelty.  In the main game, you can activate it for head tracking if you prefer to play in cockpit mode and the camera will move with your head.

The physics in this game are also incredible.  While mostly transparent to the player, you can choose to enter a telemetry mode in a replay and get a wealth of statistics about your car, including horsepower output, individual tire speed, tire temperature (outer, center and inner tire) body G forces, and the like.  If you're a physics nerd, you won't be lacking for information here.

Pictured here is the telemetry readout, which besides the engine and transmission performance on the right, is showing the amount of grip on each tire, the circles being the contact friction with the road.

Opponent AI is much better that it is in the GT series here.  Other vehicles respond to where you are, and on easier difficulty levels the other drivers will occasionally make mistakes and run off the road instead of just going slower or faster on their line like they do in GT5. 

Car damage is another thing that has been with this series from the beginning, and is only available in GT5 on certain cars.  The level of damage is able to be set from cosmetic only to full simulation, in which running in to things will not only damage the body, but also throw off alignment.  What level of damage you prefer is up to you, as you don't get penalized for turning the damage model down.  You do however, get bonus credits for turning it up.

Multiplayer offers the standard features you would expect in a racing game, but also adds a few Top Gear inspired game types which are fun to play such as car bowling and car soccer.  Another mode is Rivals mode which allows you to race against a ghost of a lap time set by other players, usually of the player's skill level.  Beating this time will award you with credits to spend in career mode.

Forza is also known for its car customization.  While the performance upgrades in both titles are about the same, Forza gives you an unparalleled amount of cosmetic customization ability, with some of the more talented members of the community creating photorealistic images of people on the sides of cars, which you can then sell on the storefront for in game cash.

The community has received some additions this time out with the addition of car clubs.  Not only does this help you find people to play the game with online, but also allows members of clubs to share cars in their garage with other members to increase the number of options you have when going up against online opponents or trying to progress in career mode.

As for criticisms, there aren't many.  The biggest one is that while they added a few environment effects, such as dynamic lighting based on the time of day and overcast weather, I'd still like to see them add rain and snow to change up the conditions of a race.  The car rating system (consisting of an overall letter as well as a numerical performance value) can sometimes be misleading when entering an event and you can be matched up against some cars that it's almost impossible to compete against.  Aside from that, though, the game is everything racing sim fans could have hoped for.

In conlusion, fans of racing games and racing sims can do no better than Forza 4.  It's a must buy if you're interested in the genre.

If anyone has any questions, or thinks there is anything that I missed, please let me know in the comments and I'll try to address them.