疑似计算错误

输入
FullSimplify[
Sum[((k*(k - 1)*4!*(-1)^i)*x^(k + i))/(k!*(4 - k - i)!*i!), {k, 0,
4}, {i, 0, 4 - k}] ==
Sum[((k*(k - 1)*n!*(-1)^i)*x^(k + i))/(k!*(n - k - i)!*
i!), {k, 0, n}, {i, 0, n - k}] /. n -> 4]
输出
x^2 == x