The fourth Brandenburg Concerto is scored for violin, two flutes, strings, and continuo. Bach's score calls the flutes "fiauti d'echo", an unusual term which probably indicates that a recorder in F was intended. But does "d'echo" mean more than simply specifying which recorders were to be used? If indeed recorders, rather than the transverse flute, then one might presume that the softer sound of the recorder is intended as a sort of echo - except for the fact that the flutes appear from the start of the Fourth Brandenburg, even without the solo violin, and while they occasionally double the solo violin, they are rarely used in an echo function. They do occasionally "echo" each other, especially in the second movement, but one wonders whether Bach really thought of these repeated "echoes" as softer echoes, or merely as imitative passages, which we find so often in Baroque music.
The flutes in fact play a prominent role, which might make one think this concerto has a solo group (concertino), in the concerto grosso format, but the work is sometimes considered a solo concerto for violin, given the long passages for solo violin alone (without the flutes and with the orchestral strings providing limited support). Or perhaps the Fourth Brandenburg is an amalgamation of the two distinct types of concertos.
The Fourth Brandenburg Concerto is unique in that it is the only of the six in which all instruments are used in all movements. (In the other concertos, the middle movements have a reduced instrumentation.) The first movement (Allegro) uses both a ritornello structure as well as an ABA form, like we might expect in a da capo aria. the ritornello theme comes back in numerous guises, including in shortened form later in the movement (typical of ritornello forms of Vivaldi, one of Bach's models for the Baroque concerto and concerto grosso), and is heard presented by the flutes and by the solo violin, but never by the strings of the ripieno.
You can see from the brief excerpt above that the orchestral strings play a lesser role thank we are used to seeing in Bach's orchestral works. Think of the other Brandenburg concertos, for example: the orchestral strings are in the forefront from measure one, regardless of which instruments are featured soloists. In any event, the soloist or solo group never begins the concerto in a leading role. Given that the flutes do have a leading role at the start, and the solo violin does not, one can be inclined to think that this is indeed a concerto for solo violin, and that the flutes are in fact not a part of the concertino, but rather a part of the ripieno - the "tutti" or accompanying orchestra. Or, perhaps Bach is simply trying something different with his concertino.
The orchestral strings help to keep the tempo by punctuating downbeats and cadences; in addition, they develop a secondary theme which emerges out of the ritornello.
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.
The repeated note figure appears frequently in the solo violin part as part of extended passage work, but also in fragments or sequences in the orchestral violins, as you can see from the example. The sequential passages are an easy way for Bach to create the impression of the music accelerating towards a cadential goal, or to help propel the music into a new key. The infusion of the arpeggiated, repeated-note motive into other themes and phrases of the first movement helps to create a sense of cohesion throughout - interesting, isn't it, that what unifies the movement in our minds is this motive from a secondary theme, and not the original theme itself, despite the repetitions of that original idea?
This opening ritornello statement, which includes both musical excerpts shown above, is exceptionally long - more than eighty measures. This, too, is unusual, for it keeps the solo violin out of the musical forefront until we are nearly one-fifth of the way into the movement. During that time, the ritornello theme is heard three times: once in G major, once in D major (dominant key), then back in G major. Although we expect an overall tonal progression of I-V-I in much music of the 17th and 18th centuries, this is the most obvious and longest move to the dominant key; there is another move to the dominant later in the movement, but it is neither as strong nor as long as this one. instead, Bach chooses to explore other related tonalities, including C major (IV), e minor (vi, the relative minor), and b minor (iii, or v of the relative minor).
The second movement is set in the relative minor key of e minor, and is sarabande-like (according to Bach scholar Michael Marissen) - moderately slow tempo, triple meter, with a tendency to accent the second beat of the measure. in this movement, the ripieno continues to play a secondary role, almost never performing melodic material on its own (there is one notable exception: the prominent bass/continuo melodic statement in measures 55-58). Meanwhile, the flutes really take the lead over the solo violin, with the first flute playing the most elaborate passages in this movement. The movement remains in the minor mode throughout, never hinting towards the relative major key of G heard in the outer movements. The effect can be seen as more solemn than somber.
"The Presto, for all its high spirits, is probably the tightest and most satisfying convergence of ritornello form and fugue in all Bach's music" (Boyd, The Brandenburg Concertos). Finally, we see the ripieno take the melodic lead:
This is a powerful, brilliant tour de force, set in duple meter, and not clearly mirroring any dance movements of the time. the lack of a dance feel or reference is likely due to the prominent given to the fugal elements here.