Granpa Jones was pulling our collective leg, I think, because I find it hard to believe that the 4-string banjo did not predate the quirkier and more specialized 5.

Pete Seeger's banjo is a 5-stringer, and if I'm not mistaken, he "invented" or pioneered a longer-necked version of the earlier mountain-music 5-string banjo. Pete plays a fairly broad range of traditional and original folk-music styles and has never restricted himself to the old-timey and bluegrass traditions; however, the constant presence of that fifth-string drone does restrict how widely a player can vary his style and repertoire. (It certainly restricts how many keys you can play in!)

Thanks to Frank and Sandy for some solid history and insight about the relatively-rare six-string banjo.