average marks of 5 students = 78
total marks of 5 students = 5x78 = 390
highest mark among the 5 students = 90
lowest mark among the 5 students = 50
since they want the highest average mark of 4 students, we remove the student with the lowest mark
total marks of 5 student - lowest mark 50
= 390-50
= 340
average marks among 4 students
= 340÷4
=85