Open dot anywhere in a graph shows that the value of function at that point is not continuous or equal to limits.

As shown in this example, as x approaches 0 the limits approach 5 but f(0) is 7.

In your case it represents that the value of f(4) is not equal to the limits (approaching from both left and right side) and is slightly larger (represented by the closed dot)

In other words, the limit exists but its value is not equal to function value