For people like me who cant apply pigeon hole concept properly.
Four of them are guaranteed to get the same grade.
Let the same grade be A and imagine the worst case.
worst case : -
Pick a student serially and give him a grade serially.
1st student A grade
2nd student B grade
3rd student C grade
4th student D grade
5th student E grade
6th student A grade
7th student B grade
8th student C grade
and so on.
repeat this till you give grade A to 4 students.
It will be 16.