Actually I'm occupied with a similar problem on same engine (see my recent topic on Q4 engine running on three cylinders).
Well, I also had a look to the Q4 manual. They say it should have 2,5bar in MAR position WITHOUT starting the engine. In MAR position the fuel pump begins to work but none of the injectors opens. At this moment there's a higher resistance in the fuel circuit. When the engine runs, fuel is evacuated by the injectors, there's a higher flow in the circuit (at the same pump performance) and pressure decreases. That seems normal to me.
Maybe your problem is another!? But what about your engine? Does it run normally?
(P.S.: Regarding the replacement of an injector, have a look on MaxiBoost's post on the topic mentioned before. He has done it once. Seems to be one hell of work!!!!)