A four piece would include drums, bass, (one of the following; piano, keyboard, organ, vibraphone, guitar) and sax.
As the four piece develops you can add alto sax, then a trumpet, then tenor sax, then a tenor trombone. Work it until you have yourself a full fledged Big Band.
The big band consist of a rhythm section, sax section, and a horn section.
The rhythm section includes drums, bass piano and/or guitar.
The saxes includes two tenors, one alto, and one baritone. Adding one more tenor and up to two more altos as you grow. (One or all of the sax players are expected to double on soprano sax, clarinet, and flute.)
The horns include one trumpet, one tenor trombone, and one bass trombone. Adding up to four trumpets and two tenor trombone as you grow.