4. Given that $P$ is a point inside square $A B C D$. If $P A: P B: P C=1: 2: 3$, then the degree measure of $\angle A P B$ is ().
(A) $120^{\circ}$
(B) $135^{\circ}$
(C) $150^{\circ}$
(D) None of the above
4. B.

As shown in Figure 3, draw $B P^{\prime} \perp B P$ such that $B P^{\prime} = B P$, and connect $P^{\prime} A$. It is easy to see that
$$
\begin{array}{l}
\triangle P^{\prime} A B \cong \triangle P C B \\
\Rightarrow P^{\prime} A = P C .
\end{array}
$$

Let $P A = k$. Then
$$
P B = 2 k, P C = P^{\prime} A = 3 k \text{. }
$$

Connect $P P^{\prime}$.
In the right triangle $\triangle P B P^{\prime}$, we have $\angle P^{\prime} P B = 45^{\circ}$ and $P P^{\prime} = 2 \sqrt{2} k$. In $\triangle P^{\prime} A P$, we have $P^{\prime} A^{2} = P^{\prime} P^{2} + P A^{2}$. Therefore, $\angle P^{\prime} P A = 90^{\circ} \Rightarrow \angle A P B = 135^{\circ}$.