How would you factor x squared plus 4?

(X^2) + 4 = 0
(X^2) = -4
X = +/- 2i

(X+2i)(X-2i)

This can't be factorised easily. The above method is the factorisation for (X^2) - 4,

and is also the difference of two squares (9-4) = (3+2)(3-2) etc...