Plectrum banjos are a 4 string banjo that t is essentially a 5-string banjo without the 5th string. The name of this banjo comes from the type of pick used to play this type of banjo - a flatpick otherwise known as a “plectrum”.
The standard tuning for this banjo is really just a drop C tuning for 5-string. Many guitar players play plectrum banjos and tune it in a tuning known as Chicago Tuning which is the same as the top four strings of a guitar.
Standard Plectrum Tuning - C, G, B, D
Chicago Tuning - D, G, B, E - same as the top 4 strings of a guitar!