Optimum general positioning of the thumb when fingers are distributed along four contiguous frets would be for the thumb to be anywhere from directly behind the 2nd finger, to somewhere between the 2nd and 3rd-- much like holding a pencil along its length between the thumb and fingers with a balance of opposition between the thumb and the fingers. In actual playing, exigencies of particular situations will have a modifying effect, but in practicing maintenance of this general position through shifts in linear passages and scalar patterns, which are typically the ground on which this is inculcated in practice, imagine that in upward shifts the thumb takes off a micro second before the fingers, and energizes the move, pulling the fingers along just slightly behind. In downward shifts, imagine the opposite-- the fingers move, pulling the thumb. In either case, the actuality will be that the entirety of the hand array moves simultaneously, which is what you want, with no component taking off before the others. But, it's a useful illusory mind-trick to keep things in line, and once the mechanics are internalized as an autonomous response, the need for indulging in the illusion goes away.