Which graph below shows the rule: output = 4 times the input

Hi there! The bottom graph is correct.

The rule: output = 4 times the input, means that the output is 4 times bigger than the input. Therefore, the graph must be very steep. In the fourth graph we can also see that quite a small input gives us a big output.

Graphs two and three are incorrect because they're not steep enough. Graph one is incorrect because it says that a bigger input gives a smaller output.

Hence, the fourth and bottom graph is correct.
~ Hope this helps you!