To determine the relationship between the parent function \( y = f(x) \) and the transformed function \( y = 4f(x) \), we need to analyze how the transformation affects the function.
In the expression \( y = 4f(x) \), the coefficient \( 4 \) indicates that the function \( f(x) \) is being vertically stretched. Specifically, every output value \( f(x) \) is being multiplied by \( 4 \), which means that the height of the graph is increased.
Therefore, the correct description of the transformation is:
The parent function has been vertically stretched by a factor of 4.