I had a short article on this but I can't find it. From memory, as RichM has said it arrived in the US with slaves and the 5 sting banjo's existed long before the 4 string banjos. I can't rmemer when frets were introduced but that was a later development. More changes occured when steel string's came into being and interest in plectrum styles of playing came about and I think this resulted in the 4 string models being produced.

The tenor banjo which is the most commonly used in celtic music and is popluar in jazz came into being around 1915. I think blue grass has it's own history and starts with Bill Monroe and Earl Scruggs sort of defined the bluegrass banjo and the required sound with a Gibson Mastertone.

Jon