YouTube
A basic guard, belonging to active guards group. It looks a bit similar to secunda custodia, but its usage is different.
Quarta custodia is always used as a reaction to the opponent's attack. The idea is to quickly retreat (and thus defend against obsessio or strike by distance) while retaining the ability to immediatelly return back with a halpschilt cut. Because the position retracts both hands, it escapes not just with the body, but also with hands. While switching into quarta custodia, you always step back, or at least shorten the stance. I.33 shows both of these possibilities; a short stance in [1r], longer stance in most of the plays.