4. Two boats departed from piers $A$ and $C$. The speed of the boat traveling from $A$ is 7 km per hour, and from $C$ is 3 km per hour. Determine after how much time the boats will meet, if the distance between the piers is 20 km, and the speed of the river current from $A$ to $C$ is 2 km per hour.
4. Since the problem does not specify the direction in which the boats are moving, we need to consider various scenarios: they are moving towards each other; they are moving in the same direction downstream; they are moving in the same direction upstream; they are moving in opposite directions. Given that the boats must meet, we can exclude the last two cases.

Case one - the boats are moving towards each other.

1) $7+2=9$ km/h - the speed of the first boat;
2) $3-2=1 \text{ km/h} -$ the speed of the second boat;
3) $9+1=10$ km/h - the closing speed;
4) $20: 10=2$ hours.

Case two - the boats are moving downstream.

1) $7+2=9$ km/h - the speed of the first boat;
2) $3+2=5$ km/h - the speed of the second boat;
3) $9-5=4$ km/h - the closing speed;
4) $20: 4=5$ hours.

Answer: They will meet in 2 or 5 hours.