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

then the value of $f(50)$ is ( ).
(A) 72
(B) 73
(C) 144
(D) 146
(4) $\mathrm{D}$ Hint: Substitute $(1-x)$ for $x$ in the given equation, we get
$$
f\left(x^{2}-3 x+2\right)+2 f\left(x^{2}+x\right)=9 x^{2}-3 x-6 .
$$

From the above equation and the original equation, eliminate $f\left(x^{2}-3 x+2\right)$, we get
$$
f\left(x^{2}+x\right)=3 x^{2}+3 x-4=3\left(x^{2}+x\right)-4 .
$$

Therefore, $f(50)=150-4=146$, so the answer is D.