Vivaldi/Bach- Concerto for Four Pianos 1st movement

This is one of the pieces currently in my repertoire, so I thought I'd analyse it to get a better understanding of it.
The piece is, as the name would suggest, written for four pianos (though originally of course it would have been written for harpsichords, however Vivaldi composed the original piece for four violins, and Bach merely arranged it for keyboards-"cembali".) Sometimes there are strings involved too, because that is how Vivaldi composed it and it was the Baroque period! The piece is in A minor, and is in 4/4. The four keyboards are constantly in interaction with each other, whether it be imitation or playing in harmony.
The structure of the piece is not clear, though at the end the first sections return. The piece begins on the dominant, with only the 1st cembalo playing single notes, then the 2nd cembalo comes in on the same notes, so that they are playing in unison, although their parts are different. They play together for a bit, then in bar 5, the other two parts come in and all the parts play chords, the tonic chord. For much of the rest of the piece, the parts all play different melodic ideas at different points, all of the ideas interchanging between parts. This creates the idea of imitation. Each part gets a solo, the 1st cembalo having the first one, then the 4th cembalo comes in, it's left hand playing in rhythmic imitation of the 1st cembalo (three semiquavers). A few bars later the other parts come back in. Then later on the 2nd cembalo has a solo, and the 3rd cembalo comes in half way through, just like the 1st and 4th cembali. However then the 3rd cembalo goes away again and the 2nd cembalo keeps playing.

I used this version: http://www.youtube.com/watch?v=4_pbxdfwi7Y The movement ends at 4:50.

Bach Brandenburg Concerto no.4, 1st Mvmt. (Allegro)

The piece is from a Concerto Grosso, a form popular in the Baroque period. It displays the typical format of the ripieno (the orchestra), playing against a smaller group of instruments (the concertino)- here, the violin and two flutes. Into the piece is written two echo flutes ("flauti d'echo"), which is quite mysterious, because we today don't know of an instrument that this could refer to. In the piece is written a main violin, but also a first violin; another of the pieces mysteries.The full concerto was transcribed into a harpsichord concerto. The concerto is one of 6, the "Six Concerts a plusieurs instruments". Bach presented them to Christian Ludwig in 1721, although they were probably composed well before then.
This piece is for all strings, continuo and two flauti d'echo. It is in G major and is in compound time (3/8). It is also at a lively tempo (Allegro). The piece begins with chords making up a perfect cadence being played by all the string parts and continuo, and the flutes playing a duet of arpeggios and sustained notes. Then the note lengths get shorter (bar 13), with semiquavers and quavers added in, the 1st violin sounding out above the other parts, at a higher pitch, or in unison with the flutes. At about 3:00, the violin part becomes heavily ornamented and it plays extremely fast patterns, generally based around scales. Sometimes the flutes have a solo part, and they play the melody line. Even though the violin part seems to be so much more complex than the flute parts, in actual fact the parts that the flutes play are advanced for the time; violins would always have had much more complex parts anyway. The flutes' part becomes more complex, however, whilst the violin accompanies them. The piece occasionally comes back to the cadential chords that there was at the start.