Arithmetic Progression proof for sum of an integer

Question

Question:-

The fourth power of the common difference of an arithmetic progression with integer entries is added to the product of any four consecutive terms of it. Prove that resulting sum is the square of an integer. 

MyApproach:

Let d be the common difference and d^4 be the fourth power.I am considering the four consecutive terms be (a+2d),(a+d),(a-d),(a-2d).What do the question mean by telling that the common difference is added to the four consecutive terms ?What is the product of the terms equals to ?

Answer 1

If the common difference is $2d$,

$$(a-3d)(a-d)(a+d)(a+3d)+(2d)^4$$

$$=(a^2-d^2)(a^2-9d^2)+(2d)^4$$

$$=(a^2)^2-2a^2(5d^2)+(5d^2)^2$$

$$=?$$