The transformed function \( y = 4f(x) \) indicates that the graph of the parent function \( f(x) \) is vertically stretched by a factor of 4. This is because multiplying the output of the function \( f(x) \) by 4 results in values that are four times larger, which effectively stretches the graph away from the x-axis.
Therefore, the correct response is:
The parent function has been vertically stretched by a factor of 4.