I agree with Aud and Sarah. The 4th is the weakeast, for anatomical reasons (I think it is because of the nerve, but I'm not sure). This is why it is so difficult to play a good trillo using it ;) 

By the way, I've studied that Schumann wasn't able to play, because he tried to use a mechanism to develop his forth finger. The result was desastrous and his finger got paralized, the only thing that was left is to compose. 

Newer theories,  though, say that there was a different reason, that Schumann's family tried to occult, distroying all the clinical documents he had in the house.