Simplifying
(x2 + -4)(x2 + -4)

Reorder the terms:
(-4 + x2)(x2 + -4)

Reorder the terms:
(-4 + x2)(-4 + x2)

Multiply (-4 + x2) * (-4 + x2)
(-4(-4 + x2) + x2(-4 + x2))
((-4 * -4 + x2 * -4) + x2(-4 + x2))
((16 + -4x2) + x2(-4 + x2))
(16 + -4x2 + (-4 * x2 + x2 * x2))
(16 + -4x2 + (-4x2 + x4))

Combine like terms: -4x2 + -4x2 = -8x2
(16 + -8x2 + x4)