If the nth term is 8 -2n then the 1st four terms are 6, 4, 2, 0 and -32 is the 20th term number
The first four terms are 3 9 27 81 and 729 is the 6th term.
5
If you mean nth term 2n then the 1st four terms are 2 4 6 and 8
2
1, 16, 81, 256 14641 is the 11th term.
6
9, 10, 11 and 12
a, ar, ar^2 and ar^3 where a and r are constants.
Each number is four more than the previous number.
29
The first four-digit Fibonacci number is 1597 - equal to 610 + 987.
a four number sequence is the easiest and longest number sequence to remember.