\[x^{4} + 64 =\]
\[= x^{4} + 16x^{2} + 64 - 16x^{2} =\]
\[= \left( x^{2} + 8 \right)^{2} - (4x)^{2} =\]
\[= (x^{2} + 8 - 4x)(x^{2} + 8 + 4x)\]