Classical composer and pianist Robert Schumann attempted to remedy the inherent weakness between the 3rd and 4th fingers by applying a bizarre contraption strapped to his hand. The end result was that he permanently damaged the fine musculature and connective tissue. Since both the 3rd and 4th finger tendons attach to a small muscle group, they tend to move together when flexing the muscles (can be easily seen when trying to move your pinkie independently from your ring finger). There is no way to overcome this anatomical weakness in human hands. The best we can do is minimize the movements. What I mean by this is don't make exaggerated movements. Make smooth, small movements and resist the urge to let your fingers "kick" up and down and dramatic motions. Try to mentally encourage ALL your fingers to remain no more than a half a centimeter above the strings at all times when they aren't fretting notes. It seems that "scalar" patterns work best for these two fingers. To gain independence, you'll first need hand strength and I recommend hammer-ons, pull-offs, and the previous patterns posted in this thread. All of these are good. An aggressive approach (not recommended) is to play everything you possibly can with your 3rd and 4th fingers as an exercise. Fatigue will set in quickly! But it's definitely good to try for a short duration in each practice session.