Remove screws from Duravit toilet seat
It looks like there should be a bolt with a hex-head that you unscrew using a spanner (wrench) to replace the hinge mounting (the hinge itself appears to be integral to the seat).


   

It's a designer toilet, designers tend to prioritise visuals over practicality. 

If you put some pulling force under part 4 (e.g. lever with a screwdriver using a small dowel as a fulcrum) it might create enough friction on the (presumed) captive nut or insert to hold it in place while you unscrew the bolt.