For 2, you also have 3, 3, 6, 3; 3, 6, 3, 3; 6, 3, 3, 3

You have to consider each die separately.

Same for the others

The total number of possibilities for 1 die is 6. For 2 dice, it's 6*6, and for 4 dice, you have 6*6*6*6 = 1296 possibilities

Ways to get all divisible by 3 is only when you get 3 and 6.

There are 5 'groups' of them that you got.

3, 3, 3, 3

3, 3, 3, 6; 3, 3, 6, 3; 3, 6, 3, 3; 6, 3, 3, 3

3, 3, 6, 6; 3, 6, 3, 6; 6, 3, 3, 6; 3, 6, 6, 3; 6, 3, 6, 3; 6, 6, 3, 3

3, 6, 6, 6; 6, 3, 6, 6; 6, 3, 6, 6; 6, 6, 6, 3

6, 6, 6, 6

This makes 16 in all.

The probability is then 16/1296

Hm... are you sure the dice is a regular dice? I think if it were a tetrahedron with 3, 6 and two other numbers less than 6, it'd be getting 1/16.