I also struggle with 4th finger vibrato. I watched a video yesterday on the site on exercises. One exercise was 4th finger strengthening. I think that if one does this regularly it will eventually help with 4th finger vibrato and one can relax the 4th finger because of the strength that has developed. Here is the exercise: With the 4th finger pluck the E string 4X, then do the same on the A, D, and G strings. You can then make it even harder by keeping the 1st finger down on each string in the same format.