how to control the horizontal position of object in a 4 units wide plane?

Hi, I am using unity for my game project in which a ball is continuously moving with uniform velocity on z axis on a plane that is 4 units wide(on x-axis ) and is of infinite length (at z-axis), i want to control its horizontal position in such a manner that when i move the joystick axis to -1 or 1 it moves to -2 or 2 units respectively from origin at x-axis and it must stay there till the axis is released to 0, and must move and stay between continuous position points between -2 and 2 with respect to joystick axis -1 and 1. Please help. Thanks in advance.