4. Given that $l$ and $m$ are the perimeter and the sum of the lengths of the three medians of $\triangle ABC$, respectively. Then $\frac{m}{l}$ must be ( ).
(A) less than $\frac{2}{3}$
(B) greater than $\frac{3}{4}$
(C) less than 1
(D) greater than 1
4. B.

Let the three medians of $\triangle A B C$ be $A D, B E$, and $C F$, and let $G$ be its centroid, as shown in Figure 4.
$$
\begin{aligned}
& \text { Then } E G+F G>E F \\
\Rightarrow & \frac{1}{3} B E+\frac{1}{3} C F \\
> & \frac{1}{2} B C \\
\Rightarrow & B E+C F>\frac{3}{2} B C .
\end{aligned}
$$

Similarly, $A D+B E>\frac{3}{2} A B$,
$$
A D+C F>\frac{3}{2} A C \text {. }
$$

Adding the three inequalities, we get
$$
\begin{array}{l}
2(A D+B E+C F)>\frac{3}{2}(A B+B C+A C) \\
\Rightarrow \frac{m}{l}>\frac{3}{4}
\end{array}
$$