First, some basic information about division. The result of dividing m by n, by definition, answers two questions: (1) into how many groups of n elements can you divide a set of m elements and (2) if you divide a set of m elements into n equal groups, how many elements are in one group. The fact that the answers to these seemingly different questions are the same is explained by the property of multiplication that its result does not depend on the order of factors, i.e., p * n = n * p.
If we assume that one ride serves a single customer, then one taxi is able to serve 4 customers. Let's break the set of 500 people into groups of 4 according to the taxi that serves them. Then, using question (1) above, we find that there are (500 customers) / (4 customers) = 125 groups, and the number of groups equals the number of taxis.