My -4 valve is mounted just below the top of the center section spar. Any fuel level below ~1/2 tank (both tanks, of course) gets the level below the level of the valve.
If you look at your setup carefully, you can get a good idea of how low the level needs to be, when looking in your fill port. Let's say the bottom of the valve is ~3" below the top of the spar. If the plane is level, that should be 3" below the top of the wing root on the outside of the plane, as well. A level from the fill port to the side of the fuselage will show you how many inches to add to that 3" figure, and the result would be how far down below the rim of the fill port the fuel needs to be.