Creating a four-string banjo can be done in most home-based workshops with only a few tools and materials obtained from hardware stores. The banjo has a history of easy construction, and plans are widely available for making most four-string varieties, including plectrum and tenor banjos. Armed with a good set of plans and enough time, you can build a high-quality four-string banjo that plays as well as the retail varieties.
Banjo Neck Design
The four-string banjo consists of two primary parts, the neck and the body. You can create your banjo neck using a solid block of maple with an ebony wood fret board attached. The differing four-string types require different neck lengths as well as fingerboard designs. For the plectrum banjo, which is played much like a guitar, you can go with a normal banjo neck length with 22 frets on the fingerboard. For the tenor four-string banjo, you'll need to shorten the neck to between 21 3/4 inches to 23 inches. The shorter tenor neck should also have fewer frets on the fingerboard, only 19.
You'll need to drill three holes in the neck above the fingerboard, to push the tuning pegs through to attach the strings through. Connect the neck of your instrument to your instrument body by using two threaded rods.
Banjo Body Design
The body of your four-string banjo is a bit more complicated to produce than the neck. You can choose from multiple body types. You can go with a closed-back banjo body, called a resonator banjo, or an open back banjo body. For both open- and closed-back banjos, your body will need to contain a wooden rim to attach all the banjo parts to, a tone ring for aid in producing the crisp sound associated with the banjo, a banjo head that makes up the front surface of the banjo and a tension ring for connecting the head. For closed-back banjos you'll also need a resonator to go over the banjo's rear opening and a flange to connect it.
Place two coordinator rods within the wooden rim for connecting the tailpiece of your banjo, and then slip the tone ring onto your wooden rim and use the tension ring to attach the banjo head. You can use drumhead materials for your banjo head. Connect the neck of your banjo, and then connect the resonator if you're going with a backed model. The resonator is a wooden chamber attached to the rear of your banjo that reflects sound within the banjo body and projects it through the front, increasing banjo volume and producing a brighter sound. Once it's all connected, mount your strings.
Installing the Strings
For mounting the banjo strings, you'll need to run your four strings through the tailpiece, over a bridge placed onto the banjo body, and then up the fret board, threading them through the tuning pegs located at the top of your banjo neck. Use guitar or violin tuning pegs for your strings. Finally, tune your strings according to personal preference, and you're ready to play.