What if Pierre is walking on( 4 , infinity )?

on the interval ( 4 , infinity ) Pierre is walking straight  ...  the graph is not increasing or decreasing

That line is horizontal (slope of 0).  He's not going uphill or downhill, so the graph is not increasing or decreasing there.
 


TRY IT:

For the following graph, list the intervals where the graph is increasing and decreasing:
 

list the intervals where the graph is increasing and decreasing
 

*Remember to answer in interval notation using only x values (no y values allowed)!