Most of the information's that are required for solving this question is already given. it is important to read the question minutely and then solving the problem will become easy.
The amount earned by washing 7 cars = $35
Then
The amount earned by washing 1 car = (35/7) dollars
= 5 dollars
So
The amount earned by washing 4 cars = (5 * 4) dollars
= 20 dollars
So you earn $20 for washing 4 cars. I hope this is the answer you were looking for and the procedure is also clear to you.