I recommend to use the third finger as a base of support for the fourth finger. This is logical as they are anatomically connected. It is silly to expect them to act independently, therefore.
Playing thirds will help to strengthen the whole hand in time, as will playing some fingered octaves. Allowing the third finger to move as if it were in use when playing simple octaves is good, also; meaning that the distance between fourth and third is maintained as a whole step for each simple octave. That would be the same position for the four fingers as the first tetrachord of a minor scale. To further explain: the third and second fingers touch at all times, while maintaining the distance of a whole step between the third and fourth fingers.
Finally, it will become immediately clear that moving the left elbow to the right will cause the fingers to move higher over the fingerboard and at the same time will cause them to curve into an arch, which we all know is a natural form of strength. This last mentioned positioning, i.e. from left to right and back, of the elbow, is extremely important to helping keep the hand and fingers in a consistent form for playing on each of the four strings. Toscha Seidel, who was of short stature, shows this principle in some of his photos. It is more obvious in short limbed players.
Take advantage of films on YouTube and watch the best left hand technique players. See how they do it, and never forget that the best model is Heifetz.
Go to the String Store