In the previous episodes we've seen how time and space can emerge from ratios of mechanical quantities. In this fourth video we show that the interplay of mechanical quantities can also produce motion. We focus on uniform motion, at a constant speed. We give a few examples of speeds that can be understood from ratios of pairs of mechanical quantities. These simple speeds are found everywhere, they can be used to describe the motion of bullets, the propagation of sound in the Earth or inside the human body, the thinning of viscous liquid threads, the sedimentation of objects, or the motility of bacteria. As soon as two mechanical quantities are on the same diagonal of slope -1 in the table introduced in the first episode, then their ratio will produce a speed.