The function below models the correlation between the number of hours a plant is kept in sunlight (x) and the height (y), in mm, to which it grows:

y = 4 + 2x

what does the y-intercept of this function represent?

the original height of the plant was 4 mm.
the original height of the plant was 2 mm.
the height of the plant increases by 2 mm for every hour of sunlight it receives.
the height of the plant increases by 4 mm for every hour of sunlight it receives.