You should get this right after you buy your car for episode 5.
If you spent gold to beat bosses and get their cars, and didn't buy a car from that episode, just used the bosses' car from the previous episode, you will need to buy a car from that episode.
You could buy the cheap crappy episode 5 car with your bills that you got from beating the episode 4 boss. I would recommend saving up for the Lamborghini Veneno (costs 750k) because you will need to buy that car eventually if you want the achievements for all getaway missions and beating the ep 5 boss. But lets face it, you made it this far, you want to complete this game so you can uninstall it and save yourself 700 MB of phone space, so save up for the Lamborghini Veneno.
Edit: MarekVit says that he unlocked the achievement without buying a car from episode 4; only by winning the boss' car.