4. Given a quadratic equation in $x$
$$
x^{2}+\left(a^{2}-1\right) x+a-2=0
$$

one root is greater than 1, and the other root is less than 1. Then ( ).
(A) $-11$
(C) $-21$

Translate the above text into English, please keep the original text's line breaks and format, and output the translation result directly. 

The translation is as follows:

4. Given a quadratic equation in $x$
$$
x^{2}+\left(a^{2}-1\right) x+a-2=0
$$

one root is greater than 1, and the other root is less than 1. Then ( ).
(A) $-11$
(C) $-21$
4.C.

Let $f(x)=x^{2}+\left(a^{2}-1\right) x+a-2$, its graph opens upwards. According to the problem, $f(1)<0$, that is,
$$
1^{2}+\left(a^{2}-1\right) \times 1+a-2<0 \text {. }
$$

Simplifying, we get $a^{2}+a-2<0$, solving this yields $-2<a<1$.