posted by Bijay .
Construct a square ABCD,length of whose one side is 4 cm.
I assume you know how to construct a line perpendicular to a given line.
On line AB, mark off 4 units. Construct a perpendicular at those endpoints.
Mark off the same 4 units on each perpendicular. Label those points C and D.
Connect C and D for the 4th side