This is easily the best Forza yet, and I would say that the series is now on par - if not better - than the Gran Turismo series (I have both Xbox 360 & PS3 so am unbiased).
PROS; * Superb graphics. The tracks are highly detailed, even including mountains off in the distance on some tracks! The cars are the stars though - they look amazing. From Autovista to on the track, the cars (and that's ALL of them, not like in GT5) look stunning. The cockpit view is now also better than ever and I find myself using it for most races, apart from cars with very restrictive views. The only thing Turn 10 (the developers) STILL haven't addressed is that as good and practical as the cockpit view is - it still doesn't look like there's an actual windscreen! There's no graphical hints at glass, reflections, dirt etc. As for the race replays, I'd say they are finally up to the high standard set by the Gran Turismo games and look great. Also, everything runs at a silky smooth 60 fps (when actually racing, I believe replays are 30fps) without a hint of any screen-tearing. This game was made for your HD TV!
* Car handling is better than ever - they really are great fun to drive. You can really feel the connection between your car and the road, more so than in Forza 3. Right now I'm loving the american muscle cars! There are loads of options as well to make the game more "arcadey" or more of a simulation - whatever you prefer. Oh...and the cars sound fantastic too - you'll want to turn your TV volume up!
* The career mode (or now World tour) is just as big as ever. The way the World tour mode is structered also means there's now much less repetition of tracks in any given year. In a typical year you'll only visit most tracks once. Online there is the new "Rivals" mode - think EA's "autolog" and you get the idea. Basically you race against a ghost of either someone on your friends list, or a random rival, on a chosen track. As with Autolog its very addictive as you constantly try to beat your friends times. All of the other usual online races are also catered for.
CONS; * Very few new tracks - most were also in Forza 3. As much as I like racing around the likes of Suzuka, Nurburgring & Catalunya these circuits have also been in just about every other racing game in recent years.
* Still no night races. C'mon guys - Polyphony digital were doing these in the first Gran Turismo on the PS1!
* No weather effects. All races are in the dry.
* No city tracks. They've even removed New York! As nice as most of the tracks are, I would like a bit more variety.
* Silverstone - it's still the old layout! F1 2011 not only has the new layout/start-finish straight, but the new pit complex as well. Also for some reason Forza 4's Silverstone seems very boring to drive on, and is one of the few races where I usually hire a driver.
* Lack of customizable options in offline "quick races". Sometimes I just want to do a one-off race (offline) on a chosen track, but I can't even adjust the number of laps! Most races here are only 2 laps. This most basic of options is a huge oversight. Update: 3 months on and Turn 10 still haven't addressed this, and to be honest I don't think they will (they never did in Forza 3).
One slightly worrying inclusion in Forza 4, is that of Top Gear soccer, and another game where you knock down skittles with your car. I really don't want to see the Forza series going down the silly minigames route. Overall though, this is a great racing game. In a year where we petrolheads have been somewhat spoilt - DiRT 3, F1 2011, Shift 2; Unleashed and now this, Forza 4 still manages to stand out. From the gorgeous visuals to the superb handling, menus that are quick & easy to navigate, this game will hook you in then keep you going for ages. Turn 10 really need to start introducing new ideas/features in Forza 5 though, including varied environmental conditions (rain, night etc) and above all...more new tracks! Whether real or fictional, the Forza series desperately needs an injection of new tracks to race on.
*** 3 month update ***
I have added a few new points to my original review above, and also just want to comment on the game roughly 3 months since it's release. I mentioned the lack of new tracks before, and this does - as expected - start to hurt the game after prolonged play. After playing this alot in my first month with it, I then went about 3-4 weeks without playing it all. I have recently got back into it though. Turn 10 have been good in releasing monthly car packs since October, but I'd rather see some new tracks. Online aside, this is a game that is best played in small doses, just doing 1 or 2 races per gaming session. This greatly increases the game's longevity, especially in career/world tour mode. I have both Forza 4 & Gran Turismo 5, and despite a few things that GT5 does better I have easily played F4 more. Why? Mainly because of the smoother graphics, and it's much more streamlined menu system - makng races quick and easy to get into (and that's taking into account GT5's version 2.0 update). Overall I stand by my original score of 4/5 and highly recommend this game.