This is not the most accurate approach by any means, but something you might try first:
1. Get all your car sprites and put them in a family, call it "cars" for example.
2. Then add instance variable to that family called "car_number" and in your layout select all 4 cars and give them unique car numbers (from 1 to 4)
3. Create a new sprite object, call it "finish_line" (if you don't have one already) and place it at the finish... line
4. Create a new text object, call it "leader" and place it where you want to have it shown on the layout
4. In the events sheet do the following:
Origin points matter in this case, so you might want to create new ones if you want further accuracy.