There must be a description of the ball and its environment, somewhere earlier on the sheet where we can't see it. We don't know what the conditions and environment are around the ball as it moves downward. So I can only answer in the most general way, and make up some scenarios.
If the ball is falling from an airplane, then it has to use some of its energy to push air out of the way, and even then, it loses some more energy as the air molecules scrape against its side. This is all called 'air resistance'. It's a form of friction, the form that takes energy away from anything that's moving through air.
If the ball is rolling across the lawn, then it takes some energy to bend the blades of grass down along its path. That's why a ball that's rolling anywhere on Earth always stops, although Newton's laws of motion say that an object in motion always keeps moving and doesn't stop.
8 miles per hour because if it is moving at 4 miles every half hour that means you have to multiply it by two to get it in miles per hour and we all know that 4 times 2 is 8 so it would be 8 miles per hour =)
