4. If $M=(|a+b|-|a|+|b|)(|a+b|$ $-|a|-|b|)$, then the range of values for $M$ is ( ).
(A) all positive numbers
(B) all non-positive numbers
(C) all negative numbers
(D) all non-negative numbers
4.B.
$$
\begin{aligned}
\text { Solution } 1: M= & (|a+b|-|a|+|b|) . \\
& (|a+b|-|a|-|b|) \\
= & (|a+b|-|a|)^{2}-|b|^{2} \\
= & (a+b)^{2}-2|a(a+b)|+a^{2}-b^{2} \\
= & a^{2}+2 a b+b^{2}-2|a(a+b)|+a^{2}-b^{2} \\
= & 2 a(a+b)-2|a(a+b)| \leqslant 0 .
\end{aligned}
$$

Solution 2: Take \(a=b=0\), then \(M=0\), which can eliminate options (A) and (C); then take \(a=1, b=-2\), then \(M=-4\), which can eliminate option (D).