The time lapse is 8 hours
To determine the average temperature you have to add the temperatures registered by the number of hours
The average rate of change is the change of value and can be calculated as

Where
(x₁, y₁)
(x₂, y₂)
are the coordinates of two points of the fucntion
Let 4pm be time zero
Between the first two points the average change of rate is

Between the second and the third

Between the third and the fourth

Between the fouth and the fifth

The greatest rate of change per hour is in the intervel 8:53a.m. to 11:53 a.m.