If you want to know what sequence the numbers form, then it is as follows:
4, 12, 5, 36, 6, 108, 7, 324, 8, 972.........etc.
First term goes up by 1, such as 4+1, 5+1, 6+1......etc.
Second term is multiplied by 3, such as 12 x 3, 36 x 3, 108 x 3 .......and so on.