Whoever told you that you had to buy all 4 new tires was just trying to steal your money. You can buy 2 new ones and put them anywhere on the vehicle you want. but if your wanting to try and match the 90% put them on the front would be the best place to get them to wear quicker. Not sure where your going to get tires, but i only go to discount tire for my tire needs. you might be able to go to them and they might have a used set of tires that are maybe 75-90% tread, for cheaper then buying 2 new ones. Ive done that before when i need a set of snow tires for when i went to colorado for spring break. Im a perfectionist so my tires would all have to be atleast the same brand and tread pattern, but you can get away with running different ones on the front and the back. Just get the same size and the same type (all-terrain, highway, or Mt. whichever you currently have)