Floris I'd rather use all 8 knobs as analog inputs and at the same time the 4 extra trigger inputs as digital inputs. Is that possible with a minijack tip-to-ground connection button?
Oh. No.
The issue is that when you reconfigure the bottom 4 CV INs to work as trigger inputs, they are wired in parallel with the 4 buttons (this way you can e.g.: send an external trigger to automate what you'd do with the corresponding button). So if you connect those "jack buttons" to the 4CV INs you'll be just replicating the 4 existing buttons, which is pointless.
What you could do, which is a bit more convoluted, is disconnect the analog ins from the bottom 4 CVs(i.e.: remove all the "analog" jumpers) and then use the jack signals available on J9, J10, J11, J12, J17, J18, J19, J20 (2 connectors per each jack socket) and wire them via a suitable resistor divider to the two spare digital pins (D10, D11) and/or some of the pins used by the LED bar.