I know the difference is big between 4:08 and 4:00, but I ran 4:08 this year just off of a couple 3-4 mile tempo runs per week. I'm not sure you need anything more than two hard tempos and maybe some 200 meter repeats after the tempos every week. Get to the point where you can run 5:10-5:15 on the tempos and you should be good to run 4:00.