y < |4-x|

First we draw the graph of the boundary equation which is

y = |4-x| 

and since < does not include equality we must draw

the graph dotted:




Next we only need to find out whether the solutions are all below
the graph or above it.  To find out we pick any test point which
is not on the graph.  The easiest test point is the origin (0,0)
and since it is not on the graph, but below the graph, we can use
it as a test poin,  So we substitute (x,y) = (0,0) in

y < |4-x|

0 < |4-0|

0 < |4|
0 < 4
That is true and that nmeans that the origin is a solution, and 
since the origin is below the graph, all solutions are below the 
graph, so we shade the area below the line, and the final graph 
is this:

  

Edwin