Mostly 4ft - particularly for the middle section. Having said that I am thinking of a raised 1ft wide strip at the back of the display. (This would be raised by 9 to 12 inches. What sort of incline can you handle?)
I'll provide some sketch ideas.
Mark
What limits the incline is the length of climb rather than the height. The angle where the incline ends and the line returns to the horizontal is pretty critical. If the change is too sharp, the loco comes uncoupled from the train.
Look forward to seeing the sketches.