The following figure shows the graph of continuous function $y=f\left(x\right)$ on the interval  $[1,3]$. The points A,B,C have coordinates $(1,1), (3,2), (2,3)$ respectively, and the lines  $l_1$ and  $l_2$ are parallel, with  $l_1$ being tangent to the curve at C. If the area under the graph of  $y=f\left(x\right)$ from  $x=1$  to $x=3$  is 4 square units, then the area of the shaded region(in square units) is: