The answer is quite easy: the MRS between two points is equal to the slope of the secant line passing for those two points.
The slope of the secant line can be found as:
Δy/Δx = Y2-Y1/X2-X1
Here we have:
Y1 = 4 (ORANGES) X1 = 6 (APPLES)
Y2 = 5 (ORANGES) X2 = 4 (APPLES)
* O = Oranges
* A = Apples
= (5-4)/ (4-6)
You used to get 2 as result because you did (X2-X1)/(Y2-Y1), you basically calculated the multiplicative inverse of the slope of the secant line passing for the points.