Weathermatic® SmartLine® SLM4 4-Zone Expansion Module for SL1600 Model, will assist you in achieving your project's zone requirements. With over 500,000 SmartLine® controllers saving vast amounts of water around the world, your next project will be added to the growing list. The SmartLine® feature set exceeds that of most high-end controllers, yet was engineered to make ET-based water management affordable for any sized project.