4. Let $f(x)$ be a function from $\mathbf{R} \rightarrow \mathbf{R}$, and for any real number, we have
$$
\begin{array}{l}
f\left(x^{2}+x\right)+2 f\left(x^{2}-3 x+2\right) \\
=9 x^{2}-15 x .
\end{array}
$$

Then the value of $f(50)$ is ( ).
(A) 72
(B) 73
(C) 144
(D) 146
4. D.

Substitute $1-x$ for $x$ in the given equation to get
$$
\begin{array}{l}
f\left(x^{2}-3 x+2\right)+2 f\left(x^{2}+x\right) \\
=9 x^{2}-3 x-6 .
\end{array}
$$

From the above equation and the original equation, eliminate $f\left(x^{2}-3 x+2\right)$ to get
$$
\begin{array}{l}
f\left(x^{2}+x\right)=3 x^{2}+3 x-4 \\
\quad=3\left(x^{2}+x\right)-4 .
\end{array}
$$

Therefore, $f(50)=150-4=146$.