I would never put a sticker on my vehicle, but if I were to...
If you absolutely had to choose from these four, I would go with the third one. The car is in correct proportion, and I think the letters are easiest to read in the white block.
If you could alter one, go with the fourth and thicken the lines in the letters. That would make it easier to read, IMO, and look better than the white block letters.