4. Given real numbers $a$ and $b$ satisfy $a^{2}+a b+b^{2}=1$, and $t=a b$ $-a^{2}-b^{2}$. Try to find the range of $t$.
(2001, National Junior High School Mathematics Competition)
(Tip: Treat $t$ as a parameter, construct a quadratic equation with $a, b$ as its roots. Use the discriminant to find the range of $t$: Answer: $-3 \leqslant t$ $\left.\leqslant-\frac{1}{3}.\right)$