I want to start off by saying I want to love forza 4. So much. I love the way it was done, and 95% of the things in the game are great, and done better than GT5, I even agree the physics are above GT5. But T10 got a few things majorly wrong, and it screws the entire game up for me.
1. Force Feedback. It's just much too weak. I play with a Fanatec GT2, FF turned up to 100 and in game up to 100. Still not half as strong as it is with the GT2 on GT5. This ruins so many things. Turn in isn't crisp, it's harder to feel when the tail starts to get loose, and it's hard to know when you are near the edge of grip. Rumble strips aren't as strong either. There's just not that "tight" feeling that you get in GT5 and most PC sims that tells you there is plenty of grip. It's extremely aggravating.
2. Steering Modes. Sim steering irks me. So does normal steering. There needs to be a happy medium. Normal steering is pretty good for regular driving, the cars feel good, until you have to countersteer, and then it just becomes way too easy. Sim steering is great for drifting and most of the time great for countersteering. I love how they have implemented the correct amount of opposite lock required to catch a big slide. That being said, some times it is overdone. You don't need full opposite lock to catch a tiny bit of power over. For regular driving however, sim steering just utterly fails, it seriously does feel most of the time that you are towing a trailer or something. Just going around a corner at normal speed, it all looks fine until you exit the corner, and suddenly you are being swung the other way. It's ridiculously annoying, and leads me to the biggest of these three problems...
3. TWITCHYNESS. It's absolutely unbelievable. I am genuinely surprised it isn't discussed more frequently. Every car, no matter the power, no matter the steering mode, takes the concentration of a neurosurgeon to drive correctly. I can't stand it. To specifically test this, I took two cars I knew (through GT5 and reading and watching real world reviews) to be good to drive, balanced and confidence-inspiring. The 458 Italia and the E90 M3. I first drove them on FM4, no aids with both normal and sim steering. Exiting EVERY SINGLE CORNER , full throttle, half throttle, no throttle, progressively increasing throttle, didn't matter, I had to make some sort of countersteering motion. Every time, both cars. They would not just drive out of the corner. The only way I could do it was to slow down to a slow, uncompetitive speed. After doing a whole lap like this, I watched the replay, and it just looked like I was taking a nice little sunday drive! I couldn't push either of the cars at all without massive understeer or snap-oversteer. Then taking both cars to GT5, this problem did not exist. No aids on comfort softs, and they were stable as could be as long as I didn't drive like a maniac. But give the cars a flick, or a power over, and the tail stepped out, just as I would expect them to do. Watching fifth gear's review of the 458 Italia confirmed what I believed. They specifically said the car was "stable and inspired confidence". That's not at all how it was in forza.
I'm not alone in this belief of twitchyness. In the German comparison of FM4 vs real life with an M5 (no aids, wheel, sim steering) the reviewer specifically said that you couldn't get near the throttle mid corner like you could in the real M5, and that the car was unstable and hard to correct.
I really like FM4 as a whole, but they got the most important thing wrong. The actual feel of the driving. Even the physics are great, there is just something wrong with how the input gets from the driver to the game, and how the feedback gets from the game to the driver.