Finding Decreasing Intervals: Parabola with Vertex at x = 4

To solve this problem, we'll consider the following:

• Understand that the vertex form of a parabola, located at point $x = 4$, gives critical information about its symmetry line and vertex location.
• The parabola can either open upwards or downwards, determined by the sign of $a$ in the quadratic equation $y = ax^2 + bx + c$.
• If $a > 0$, the parabola opens upwards, meaning it decreases for $x < 4$ and increases for $x > 4$.
• If $a < 0$, the parabola opens downwards, meaning it increases for $x < 4$ and decreases for $x > 4$.
• Given only the vertex $x = 4$, we lack the information about the parabola's opening without the value of $a$.

Therefore, it cannot be determined whether the function is decreasing on any specific interval without knowing the sign of $a$.

The correct choice is Cannot be determined.