If I'm interpreting your problem correctly, something is wrong in your problem.

Consider the sequence:

a-3d,a-d,a+d,a+3d

This is an arithmetic progression with constant difference 2d.

The product of the 2 outer terms is a^2 - 9d^2. The product of the 2 inner terms is a^2 - d^2.

Your requested ratio is:

which leads to

which is impossible.