Magnetic fields spinning diskWatch
So far I can only think of a explanation without the use of a mathematical formula and I don't know if that is required for 4 marks:
The higher the speed the higher the rate of change of flux so emf is greater so reduce speed more
The lower the speed the slower the rate of change of flux so emf is less so reduces speed less
So as speed slows down the rate of change of speed decreases
For it to decrease exponentially it must decreases at a rate proportional to its current value
If you want to make the explanation more quantitative, you could mention that Lenz's law relates induced emf linearly to rate of change of flux (or rate at which flux lines are cut).