HP ProDesk 400 G4 Maintenance And Service Manual page 51

6. Remove the eight Torx-15 M3.3×8.9 screws that secure the system board to the computer chassis.
7. Remove the computer chassis insert (1) from the front edge of the computer.
8. Swing the front edge of the system board (2) up and back until it rests at an angle.
9. Remove the system board (3) by sliding the front edge up and forward at an angle.
Reverse this procedure to install the system board.
System board 43