4. Let $a=\sqrt{5}-2$. Then the value of the algebraic expression
$$
3 a^{3}+10 a^{2}-12 a+6
$$

is ( ).
(A) 4
(B) 6
(C) $6-\sqrt{5}$
(D) $6+\sqrt{5}$
$$
\begin{array}{l}
\text { Given } a=\sqrt{5}-2 \Rightarrow a^{2}+4 a-1=0 \text {. } \\
\text { Then } 3 a^{3}+10 a^{2}-12 a+6 \\
=\left(a^{2}+4 a-1\right)(3 a-2)-a+4 \\
=4-a=6-\sqrt{5} \text {. }
\end{array}
$$