x >= 0 & y >= 0 means the feasible region is in 1st quadrant.
Now, our equation is x + y <= 4
Let us take Origin (0, 0)
So, our equation becomes
0 + 0 <= 4
0 <= 4
Which is true,
Therefore, Origin (0, 0) lies in the feasible region. So, we shade the left side of the line.
Left side of line x + y <=4 and 1st axis makes the feasible region as marked in the graphic.
I suggest you look at the topics of Chpater 6 Class 11 Linear Inequality http://teachoo.com/4572/1617/-Example-9---Solve-3x---2y---6-graphically---Chapter-6/category/Graph---1-Equation/
Please check all questions in the list also.