The solution would be like this for this specific problem:
4/13 * 3/12* 2/11 * 1/10 = 0.0014
So the probability that the four horses owned by one person finish first, second, third and fourth is 0.0014. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.