Expand the first part twice with a negative sign between and substitute $\left(−b\right)$ for $b$ in the second part. Carefully distribute to simplify.

$\left(a^{4} + 4a^{3}b + 6a^{2}b^{2} + 4ab^{3} + b^{4}\right) − \left(a^{4} + 4a^{3}\left(−b\right) + 6a^{2}\left(−b\right)^{2} + 4a\left(−b\right)^{3} + \left(−b\right)^{4}\right)$