Assuming the mass of the container holding the 4 blocks is negligible, we are attempting to accelerate 4 times the mass (which would decrease acceleration by a factor of 4) with 2 times the force (which would increase acceleration by a factor of 2). So the resulting change in acceleration will be 1/4 * 2 = 1/2 as much as the original situation. And since the original situation was an acceleration of 1 m/s^2, the situation with 2 newton's of force and 4 blocks will be 0.5 m/s².
