There are four balls: red, yellow, blue, and green, with the same size and different weights. Take a balanced, place the red and yellow balls on one end of the balance, and place the blue and green balls on the other end of the balance. As a result, the balance remains balanced. If you place the red and green balls on the left end of the balance and the yellow and blue balls on the right end of the balance, the left end sinks. If you place the red and blue balls on the left of the balance and the yellow ball on the right, the right sinks. Based on this, judge the order of the four balls from light to heavy? ['Blue, red, green, yellow.', 'red, blue, green, yellow.', 'blue, red, yellow, green.', 'yellow, blue, red, green.']