The transformed function \( y = 4f(x) \) indicates that the parent function \( f(x) \) has been vertically stretched. When the coefficient of the function (in front of \( f(x) \)) is greater than 1, as it is in this case (4), it indicates a vertical stretch.
Therefore, the correct description of the transformed function is:
The parent function has been vertically stretched by a factor of 4.