Lets say you agree to meet someone, you tell them to come to a certain place at certain coordinates given by (x, y, z) or latitude, longitude, and height from say, the water level. But that's not enough, you need to specify a time, that than becomes the 4th coordinate. Furthermore, given any dynamical system time is a parameter telling you at which stage of evolution the for mentioned system is in. What is the actual physical 4th dimension?