7. Let $x_{i}(i=1,2,3,4)$ be real numbers, and $\sum_{i=1}^{4} x_{i}=\sum_{i=1}^{4} x_{i}^{7}=0$, find the value of the following expression $u=x_{4}\left(x_{4}+x_{1}\right)\left(x_{4}+\right.$ $\left.x_{2}\right)\left(x_{4}+x_{3}\right)$.