What about for n=4? I think the issue is that you're supposing that 2^n-1 is already prime. Primes of this form are called Mersenne primes if you want more information on them.
What about for n=4? I think the issue is that you're supposing that 2^n-1 is already prime. Primes of this form are called Mersenne primes if you want more information on them.