My take opinion on things.
But first
also (FM4 physics are still a mystery to me) although i liked the physics in F3
will be purchasing a elite along with F4, gonna play the hell out of it, and just switch it between GT5 and forza4 just like i did with forza 1-2 and Gt4. same thing different year, Bring it on T10.
I'll mostly be doing painting(replicas) in F4, that's all i did in F1, but i will still finish out the seasons and championships and all that good stuff, (will probably end of purchasing a 911 pure..)
If i want to have fun on forza ill just pick up the controler and drive, But GT will still be my prime racing game, due to the fact of my G25.
But i guess when comparing the 2 games, each does things extremely different.
forza4 car list is smaller, but trumps the GT5 1000+ list although i am a standard lover.
intern GT5 trumps forza 4 gameplay due to the fact that you don't just have to race on a "dry" course, you can race on dirt, snow,wet tarmac, rain soaked tarmac, and the physics engine is impressive on either one (Give or take dirt physics which are a hit or miss, but they are there, And light years ahead of GT4 dirt physics, I especially like the ice and fresh snow physics.
Forza4 trumps GT5 on customization and livery(is just always amazing) many different styles you can create with the cars, many replicas...things i absolutely adore in games.
But again GT5 does other things that trump forza 4, like the aforementioned weather conditions and actual day to night mode, or set night mode SS5-7, Alot of little details like dynamic tire smoke, rain spray, dirt dust and rooster tails that interacts with different light sources, and shadows from trees and cars and other obstacles,Different headlight models for cars, HID, standard halogen etc, and how they vary from actual light output, how doing burnouts and drifting actually affects the heat on your tires when they smolder for a bit, and the list goes on.
In a technical aspect, GT5 trumps Forza, imo, it does more actual things concerning actual gameplay, I mean if you want to drive a Ferrari 458 italia at night around the ring in the rain you can do that, it's just nice to know their are different conditions that will affect your driving, one minute dry...some minutes later light rain, medium pour then heavy rain, and your still using sports hards and you feel the car try to adapt with the changing environmental conditions, GT actually just goes way farther then Forza(so far) in-terms of core play gameplay and situations
But as an overall car experience, (car life style) i would choose F4
As an overall racer and having a good time actually driving cars in different conditions i would choose GT5.