More a valuable information to avoid wasting money than a guide (i dont really think this game needs a guide anyway).
The turf 4 boss has 1.240 power, so you need the turf 3 boss car (you need gold for this one) or the Pagani Zondra R ($600000).
I suggest you (unless you have a lot of gold to spare) to get the Pagani, becouse you will WANT the turf 4 boss car (again you need gold for this one) to grind enough money in turf 5 (where the rewards are higher) to buy the lamborghini veneno needed to beat the last boss.
Extra info: The turf 5 boss has 1.660 power in case you want to do your own maths.