Two blocks, $4.0\ \text{kg}$ and $2.0\ \text{kg}$, sit side by side on a smooth table. A horizontal force of $18\ \text{N}$ pushes the $4.0\ \text{kg}$ block, and both move together. What force does the $4.0\ \text{kg}$ block exert on the $2.0\ \text{kg}$ block?