what is the torque required by stepper motor to hold an object of 400 grams
Torque is force times the lever length. 

Let's say you have a wire attached to the object, and the wire is wound around a pulley which is mounted on the motor. In that case the torque you need to hold it steady is 4N*r where r is the radius of the pulley in meters.

A big pulley means you need a lot of torque, a small pulley requires less torque, but the motor will have to spin faster to get the same linear speed.