A biquadratic form $\sum_{i,j,k,l}b_{i,j,k,l}x_{i}x_{j}y_{k}y_{l}$， how to determine whether it is positive-definite？

A necessary and sufficient condition？

In fact, I have a matrix $B=\sum_{1\leq i,j\leq n}A_{i,j}z_{i}z_{j}$. $A_{i,j}$ are $n\times n$ matrix, $z$ is n-dimension vector. I want to prove that B is positive-definite. I know $A_{i,j}$ are positive-definite, but the big matrix $\{A_{i,j}\}$ is not positive-definite. How can I prove that B is positive-definite.

If there is not a general conclusion, how about $n=2$?