Exercises to prevent 4th finger from pressing into third
I've developed a bad habit of letting my fourth finger press against my third finger when it isn't in use, this isn't a problem until i need to play quick notes involving the fourth finger. I've tried just doing slow scale fragments involving the fourth finger, but the second I lose concentration and focus on another aspect of play I notice that my fourth squeezes back against my third.
Hard to say something without seeing you play, but this could be a result of a LH-position that's too much oriented toward the first finger. You could try a 'fourth-finger-friendly' position. Place your fourth finger the way you want it, and then work your way back and let the other fingers find a comfortable position without changing the position of the fourth. You might have to stretch your first finger back a little more than you're used to, especially if you have small hands. You could give the first two pages of Schradieck a try with this idea in mind.
Agree with Stefan. What does your teacher say? If you want more feedback, a video/picture would help.