After 4 seconds, 40m/s (10 m/s x 4) is the speed of the ball which was dropped at 10 meters per second.
A free falling object is an object that is falling under the sole influence of gravity. Any object that is being acted upon only by the force of gravity is said to be in a state of free fall. There are two important motion characteristics that are true of free-falling objects:
Free-falling objects do not encounter air resistance.
All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s (often approximated as 10 m/s/s for back-of-the-envelope calculations)
Objects that are said to be undergoing free fall, are not encountering a significant force of air resistance; they are falling under the sole influence of gravity. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass.
Learn more about free fall at PhysicsClassroom.com.
Use this calculator to calculate how fast you're moving after falling a certain distance — your free fall speed.