4. Point $B$ lies on segment $A C$ of length 5. Find the distance between the midpoints of segments $A B$ and $B C$.

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$\triangleright$ Point $B$ can be chosen in different ways. For example, let it divide segment $A C$ into parts of 1 cm and $4 \text{~cm}$. The midpoints of these segments ($A B$ and $B C$) are 0.5 cm and 2 cm away from $B$, respectively, totaling $2.5 \text{~cm}$. Trying other positions of point $B$, one can verify that in all cases the distance between the midpoints of segments $A B$ and $B C$ is 2.5 cm. Why? The reason is that this distance is composed of half the lengths of segments $A B$ and $B C$. Together, these segments total 5 cm, so their halves together make up $2.5 \text{~cm} . \triangleleft$