What exercise would you recommend for the training of the fourth and the fifth fingers?
Any collection of Etudes is sure to contain some that are devoted to the training of those two fingers. In the Cramer Etudes (Bulow's selection) you will find Nos. 9, 10, 11, 14, 19, 20 adapted to your case, but do not pin your faith to the print! In all matters of art the "how" is of far more consequence than the "what." Play what you will, but bear your weak points in mind while you play. This is the real remedy. Keep hand and arm as loose as you can while training the fourth and fifth fingers.
The Action of the Little Finger
In making wide skips in which the little finger strikes a single note, as, for instance, in left-hand waltz accompaniments, should one strike on the end of the little finger or on its side; and should the finger be curved or held more or less flat?
The little finger should never strike with its side. It should always be held in its normally curved condition, and straighten at the stroke only on such occasions when its own force proves insufficient and requires the assistance of the wrist and arm muscles.
[10. WEAK FINGERS, ETC.]
To Strengthen the Weak Finger Use It
How can I strengthen the little finger of my right hand? I avoid it in playing, using the next finger instead.
By employing your little finger as much as possible and at once quitting the habit of substituting another finger for it.
The Weak Fingers of the Left Hand