If you did n exams before getting the 100, the new grade will be:
G = (84*n + 100)/(n + 1)
Particularly, if you did n = 3 exams, then your new grade is:
G = 88, and this is an increase of 4 points.
How much will a 100 bring up a grade that is 84?
Usually your grade will be the mean of the points that you got in each exam.
So, if you did n exams, and your current grade is 84, and now you get a 100, your new grade will be:
G = (84*n + 100)/(n + 1)
Now, we don't know how many exams you already did, but let's assume a total of 4 in the whole year.
Let's say that in 3 exams you have a grade of 84, and in the fourth exam you have a 100, then your final grade will be:
G = (3*84 + 100)/4 = 88
In this case, increases by 4 points.