Interesting question,..by 4th finger do you mean the 'a' finger (or ring finger)?
Butterfly picks will probably sound a lot different than your actual nail, depending on your technique. Do you hear that happening?
I know a lot of people who use glue on nails, which can be better at simulating an actual nail sound. Some people feel they work better than actual nails, but they come their own set of problems,..like popping off in performance. Real nails have also been known to break in performnace so I guess that risk is always present.
You're not alone with the thumb nail problem, and many people deal with it by just using flesh tone. Bass notes can actually benefit from the alternative timbre, but not always...
You might try checking your local nail salon to see what they have to say.