arr[a+4] - this retrieves the value at index a+4 of the array.
so if a is 10, then arr[a+4] will retrieve the value at index 14 of the array.
arr[a]+4 means retireve the value at index a of the array and add 4 to this value.
So again if a had the value 10, then arr[a]+4 will retrive the value at index 10 of the array then adds 4 to that particular value.
As you can see, they are two different actions- (also if you remember that the array subscript operator  has higher precedence than the + operator you can immediately see that they are different actions )