Banjo history is about as reliable as any other form of folklore (I've gotten in trouble more than once here by relying on folk songs for my history), but most of what I've read states that the 5th string was added in 1830 by Joel Walker Sweeney, an American minstrel musician. I'm sure the 4-string predates that considerably. Whether the early 4-strings were played with a plectrum or what, I don't know.