In order to celebrate Beijing's successful bid to host the 2008 Olympic Games, people in a certain area dressed up as cities and took to the streets to celebrate. A math teacher saw a red light hanging on the local seven-story tower, so he recited four poems: "There are seven floors of the Huoshu Yinhua Tower, and the red light on each floor has doubled. * * There are 508 red lights. How many red lights are there on the fourth floor? " This is an interesting problem, please try to solve it.