To maintain an object moving at a constant speed in a straight line, you don't need to apply any net force, according to Newton's first law of motion (the law of inertia). If there are no external forces acting on the object—such as friction or air resistance—then it will continue moving at that speed and in that direction indefinitely.
In your case, if the 2-kilogram object is moving at a speed of 4 meters per second (not milliseconds, as speed is typically expressed in meters per second or kilometers per hour), and there are no opposing forces, the net force required to keep it moving at that speed and in that direction is 0 newtons.
However, if there are forces acting opposite to its motion (like friction), you would need to apply an equal and opposite force to maintain that speed. If you provide additional details about possible opposing forces, I could help calculate the required net force under those conditions.