The first bar of variation 4 begins with an arpeggiation that is filled in with a passing-tone (goal oriented). This patterns is very common:
This is followed by a goal oriented diminution that leads up to A. However, Vivaldi leaps down a seventh instead of going up to the expected A. Here we see how free and creative Baroque composers were in their implementation of diminution.
The second bar continues with a similar arpeggiated figure that is “filled in” with passing tones and neighbor tones.
In measure 3, we once again see a goal oriented diminution that appears to be leading to nowhere, however it drops from G to C. There are couple of reasons this works. The first, is that we had heard this pattern already (measure 3 is identical to measure 1). Any diminution that has already been used, is immediately more likely to make sense to a listener as it becomes a ‘motive.’ The other reason G-C works is that it is the relation of the bass clausula leading the music convincingly (though very temporarily) to C major.
Measure 8 of this variation shows a wonderful use of goal oriented diminution. After many measures with mostly eighth notes, the series of sixteenths provides renewed energy.