Most people understand 3D so I will try to explain 4D.

Someone came up with the idea of the motor on a propeller driven plane that could rotate clockwise and counter clockwise. Whis this in place you can fly straight down and when you are ready, reverse the direction and back back up in the air. I have seen a video of this and it is cool, just before the crash can happen, the plane backs away from the ground and back into the air. It is really cool looking.