As in the Fifth Brandenburg Concerto, the opening theme of the Fourth Brandenburg is an example of the kind of theme known as Fortspinnungtypus. This kind of theme usually contains three parts: 1) the theme clearly establishes the tonality (which occurs here through the elaboration of a G major triad); 2) the actual Fortspinnung (continuation) moves away from establishing the tonality, and frequently uses sequence (what you see in the repeated-note arpeggios above); and 3) a conclusion, which contains a clear, strong cadence.