The difference is that the long-necked banjo of Pete Seeger is a five-string and the long-necked four string banjo is also called a plectrum banjo which is tuned like the five string but without the fifth string. Both are referred to as long-neck banjos but they serve a different function. 5-string long neck was invented by Pete Seeger who added two additional frets so that the lower register of the instrument could be used. The Eddie Peabody style long-necked plectrum four-string banjo was popular in Vaudeville and in 1920's jazz bands.

The plectrum long-neck was also designed to play with tenor banjos in a duo. The two compliment each other. As far as I know, the plectrum has not. . been used in Irish music. The tenor banjo with a dropped tuning, GDAE instead of CGDA is used instead. GDAE would correspond with the mandolin tuning although it is one octave lower. The CGDA matches the viola tuning.

The additional two frets added by Pete helps the banjo to accompany singing, giving a guitar-like support to the voice unlike the bluegrass banjo which requires other instruments to carry it.

Frank