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I am aware that for Viola, it's recommended to not use 4th finger as much as one would for the Violin, because of reasons such as the pinky is not naturally strong enough to press down the string by itself.
What I was wondering is what exactly the fourth-finger differences are between Viola and Violin. I know the Suzuki Viola method has the student use 4th finger almost like a Violin, minus the stretches. (3rd finger on D, and then Stretch the 4th finger to F, or things like that). I am unsure about other methods. From what I have seen other professional violists do, is that they use the first 3 fingers most of the time, and shift a lot, but they only use 4th finger in occasions such as if the note would go to fast, and there isn't time to shift, or if the 4th-finger note was just a half-step from the 3rd finger. Is that generally how using 4th finger playing Viola differs from using 4th finger while playing Violin? How would it be different if that's not the case?
I am aware that usually violists balance their thumb across the middle finger or even ring finger. My hand can stretch pretty far, so that's not necessary for most of the time. My viola-major friend told me that bringing the elbow in more helps with 4th finger.
I'm a violist, and I have the smallest hands of any adult I know, about the same size as those of an average 11-year-old. I can still manage 4th-finger stretches most of the time on a violin, but on viola I only use 4th finger stretches in 3rd through 7th positions. (Below 3rd position, the notes are too far apart; above 7th position, it's hard to stretch with the 4th finger.)
In general, the viola uses internal extensions more than external ones, i.e. stretching with the 2nd and 3rd fingers. If it's not inconvenient, I prefer to shift rather than use 4th finger. When in higher positions, if I have time, I may even stretch with the 3rd finger rather than use the 4th finger in its normal position, because the 3rd finger can actually reach farther.
That said, if you absolutely must use the 4th finger, consider placing your 4th finger first and then stretching back with the 1st finger. That balances the hand differently and gives you a little bit more range.
Kim Kashkashian has some useful thoughts on the subject in the video I posted here: