The electric force acting on the charge is given by the charge multiplied by the electric field intensity:
where in our problem
and 
, so the force is
The initial kinetic energy of the particle is zero (because it is at rest), so its final kinetic energy corresponds to the work done by the electric force for a distance of x=4 m:
where in our problem
The initial kinetic energy of the particle is zero (because it is at rest), so its final kinetic energy corresponds to the work done by the electric force for a distance of x=4 m: