Answer :

apologiabiology
note: (x)(y) means x times y

this is the difference of 2 perfect squares
(x^2)-(y^2)=(x+y)(x-y)

so
x^4-16=(x^2-4)(x+4)
x^2-4 is also the difference of 2 perfect squares
x^2-4=(x-2)(x+2)

the answer is
(x-2)(x+2)(x+4)