Hi Reetika, 

Calculation of best four score is different for each course and some subjects don't even consider four subjects. I'm giving you this link to an information bulletin that contains all details of how to calculate best four for different courses.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.du.ac.in/du/uploads/Admissions/2018/UG/UG_Bulletin2018Final.pdf&ved=2ahUKEwjgzYS02sLiAhXCZSsKHST-DE4QFjABegQIBhAB&usg=AOvVaw36w8zqM0o1ZYhySIHjGjRu

Hope this helps!