I'm going to add to this post if I see/learn more about this. Also I've been looking for a way to weaken the auto position but haven't found it yet.
The auto position is based on many, many things and unless the makers of topspin 4 tell us all of them we will never know. However I did figure out recently that if you tap/release a shot button without pressing a direction on the dpad/analog at that same instance you get better auto positioning when the ball comes back without doing anything more. Also your player gets back on balance faster if he had to stretch and your shots are better. So when the ball is coming at you the order is:
1. Press the direction of where the ball is going
2. Once your player is running toward the ball let go of the dpad/analog.(in other words don't press any direction)
3. Release/tap a shot button
4. Now pick the direction you want to hit the ball.
Depending on how the opponent hits the ball at you the timing between the steps changes, especially between 3 and 4. This may sound easy, and it is when you aren't dealing with we'll hit balls at you. And ts4 is known for its long rallies so if you screw up and are holding a direction when releasing you really fall behind in the rally.