There are 6 numbers on each die.
The total number of out comes is 6 x 6 = 36.
To get a sum of 4, the combinations are: 1 & 3, 2 & 2 or 3 & 1.
To get a sum of 8, the combinations are: 1 & 7, 2 & 6, 3 & 5, 4 & 4, 7 & 1, 6 & 2, 5 & 3
The total of those combinations are: 3 + 7 = 10 total combinations of a sum of either 4 or 8.
The probability of either is 10/36, which can be reduced to 5/18.