This is because the less starch concentration, the more water molecules exists. Since 4% is less than 10%, so the 4% will contain more water. Water molecules tend to flow from a region of higher water potential to a region of lower water potential through the semi permeable membrane in osmosis. Therefore, since the 4% contains more water, the water there will flow away from the 4% and to the 10% one in order to reach equilibrium.
