The 1920's were more the hey-day of the tenor banjos, as used in the jazz bands; Seeger suggests they got their start in the earlier ragtime era. I got my first Thoroughbred S.S. Stewart banjo from a 1920's banjo man back in 1964; he had no use for 5-string banjos and was willing to reduce his clutter for $55. I'm no longer sure where I ran my reference to 4-string plectrum banjos.