4. If the lengths of two sides of $\triangle A B C$ are $a$ and $b$, then the area of $\triangle A B C$ cannot be equal to ( ).
(A) $\frac{1}{4}\left(a^{2}+b^{2}\right)$
(B) $\frac{1}{2}\left(a^{2}+b^{2}\right)$
(C) $\frac{1}{8}(a+b)^{2}$
(D) $\frac{1}{4} a b$
4.B.
$S_{\triangle A B C}=\frac{1}{2} a b \sin C \leqslant \frac{1}{2} a b$
$$
\leqslant \frac{1}{4}\left(a^{2}+b^{2}\right)<\frac{1}{2}\left(a^{2}+b^{2}\right) .
$$