The transformed function \( y = 4f(x) \) means that the output values of the parent function \( f(x) \) are being multiplied by 4. This results in the graph of the function being vertically stretched because each point on the curve is moved away from the x-axis by a factor of 4.
Therefore, the correct description of the transformed function \( y = 4f(x) \) is:
The parent function has been vertically stretched by a factor of 4.