i'm sure that you CAN do a sub 4 using a three or four day training schedule, but you'll be a better man/woman than i...
the thought is that for carthorses, the sub 4 don't come easy and needs some real grit and application. for me that was the 5 day schedule.
now then, given that my longest run since april 13th has been 2.5 miles, there's a bit of distacne to travel for my dear self (ahem)!!