According to the properties of geometric series, we can get the power of A (n+4) = A4 * Q (the n in the upper right corner of Q cannot be typed, sorry).

So a4 = n power of a4 * q

So the n power of q = 1.

Consider the case where n is odd or even.

If n is odd, then q is1; N is an even number and q is plus or minus 1.