input: Write a python function to calculate the average grade of four students.
  assert average_grade(85, 90, 75, 95) == 86.25
  assert average_grade(100, 90, 80, 70) == 85.0
  assert average_grade(60, 70, 80, 90) == 75.0
output: def average_grade(grade1, grade2, grade3, grade4):
      return (grade1 + grade2 + grade3 + grade4) / 4