4 Three fair six-sided dice, each with faces marked \(1,2,3,4,5,6\), are thrown at the same time, repeatedly. For a single throw of the three dice, the score is the sum of the numbers on the top faces.
- Find the probability that the score is 4 on a single throw of the three dice.
- Find the probability that a score of 18 is obtained for the first time on the 5th throw of the three dice.