What event begins the 400 years?
The 400 years begin when Ishmael is sent out, not when the journey starts from Haran. At that point Isaac becomes the prime object and the central figure, since everything then revolves around him. This is distinguished from the 430 years counted from Haran.
Les explains the timing by contrasting the possible starting points. He asks whether one starts from Haran or from when Isaac was five years old. He then states that there are 430 years from Haran, but 400 years from the time Ishmael is sent out, when Isaac becomes the prime object of affection. From that point, everything revolves around Isaac.