Hi Suman,

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/

and 

http://teachoo.com/4575/1618/Example-12---Solve-x---y--=-5---x---y--=-3-graphically/category/Graph---2-or-more-Equation/

Please check all questions in the list also.