Four people who police said attacked a UW-Madison Ph.D. student who was walking in downtown Madison Tuesday night were arrested Saturday. One of the suspects is 15 years old.Madison police said the fifth-year doctoral student was walking in the 400 block of West Gilman Street at around 10:15 p.m. Tuesday when he was allegedly punched by a group of men. The men then kicked and punched him after he fell to the ground.