At 65 years, I now experience picking the 4th string accidentally when playing up the neck (ie Sally Goodin). Shortening the blade of the thumb pick does not help. Respacing strings or radiased fingerboard–what are the solutions for this thumb pick problem?
Hitting the wrong string is a problem that does not suggest to me a cure in terms of gear. So I would not call it a “thumb pick problem. The path of the thumb is not correct, and that’s what needs fixing.
With problems of this sort, I make exercises out of the problem section, repeating over and over and making sure to ONLY play it correctly, no matter how slowly. Establish the *good* habit, and build up speed, making sure it doesn’t backslide. When it seems to be working correctly, put it into context with a larger section, especially including the part *before* the problem section. Make a loop exercise out of that (if you’re unfamiliar with my Loop Exercise method, see my web site under Instructional, the Doc’s Prescriptions), and build it up to speed with NO mistakes. If you backslide, go slower until it’s very consistent, then raise the bar again.
Keep this up till you get it consistently right at the proper speed. Repeat as often as necessary until at some point the improvement will “stick”.
Without seeing you in action (and probably even if I did), that’s about the best I can suggest.
Best of luck!