4. Let the set $M=\{u \mid u=12 m+8 n+4 l, m, n, l \in \mathbf{Z}\} . N=\{u \mid u=20 p+16 q+12 r, p$, $q, r \in \mathbf{Z}\}$, then the relationship between $M, N$ is $\qquad$ .
4. $M=N$ Since $u=20 p+16 q+12 r=12 r+8(2 q)+4(5 p)$, then $N \subseteq M$, and $u=12 m+8 n+4 l=20(l+n)+16(-l)+12(m-n)$, then $M \subseteq N$, thus $M=N$.