So what does this seal do - Part #4 in image? Does it influence retaining air in the strut? Can air leak past it?

 

On the rear strut there's a 4mm nylon hose( color coded band )going in to the first brass connector with an olive cutting into the hose so it does not come out on its own, with an O ring on on the bottom connecting to a brass pressure retention valve just below that lets air go both ways but keeps around 4 bar inside the airsuspension strut , so the airbag does not collapse completely if car would go down.
electric plug in the middle of the strut is 5v 400herz and its connector giving info going to the ASU airsuspension module behind rear seat .
this one regulates and adjusts airpressure over the valvebody.

the front has a 6mm

 

The only other thing is that foam in the middle wich is just for mounting purpose to the cars body