te question is 1, 2, 4, 8 not 1,2,3,4

I think you misunderstood my previous answer

2^n-1  for n = 1, 2, 3, 4.....

Note ....if we put 1 into this for "n" we get

2^1-1 = 2⁰ = 1

And if we put 2 into it, we have

2^2-1 = 2¹= 2

etc.