why does the function k(f(x)) provide a better model for the scatter plot when 4

Answer:
It reduces the distance from the plotted points to the function curve.
Step-by-step explanation:
The goodness of fit of a model is measured by the "residuals", the differences between the given points and the modeled points. The smaller the residuals, the better the model. Any change to the model that will put it closer to the plotted points makes it a better model. The change proposed in your problem statement does that.