Hi thrasherluver,
We have a specific guide out for this game, specifically written for issues like this, and also created by yours truly. The guide can be found here, just click the link to gain instant access to it:
LinkBe sure to download it for a future reference!
Anyways, I will answer your queries here as well.
1) The markers do not have a set pattern. They are completely random and there's no telling which is what. You just have to pray for luck and hope you've got the right one.
2) Stage 4 upgrades do not come later on in the game at any point. They can only be unlocked via the markers, or using cheats.
3) Once you have assigned a car to a specific race type, and custom-tuned it for that race type, you cannot switch the car's allocated race type without losing the blueprints that you have made. For example, if you have a Corvette for drag, custom tuned etc, and want to change to drift, you can do so but you will have to start out with a stock Corvette.
4) I personally draw a line between my "favourite" cars and the "best" cars. Because I settle for nothing less than first place, I always choose the best cars. After playing the game over 10 times, I can conclude that the following list is quite simply the best car list in the game:
Grip: Porsche 911 Turbo
Drag: Shelby GT500 (2007)
Drift: Mazda RX-7/BMW Z4 M Coupe
Speed: Audi RS4/Lamborghini Murcielago LP640 (depending on how much money you have)
There are also many other factors which influence your race, and the guide explains that thoroughly.
I hope this helped you.