A lot of our customers run fairly big facilities that need more than 16 zones. If you just want to control sprinkler solenoids and have no need for sensors or wired Ethernet it’s certainly possible to compile a version of the firmware to control 4 zones. But our controller is designed to accommodate two independent sensors and support wired Ethernet, so there aren’t even 4 spare GPIO pins left on ESP8266. You might be thinking moving to ESP32 saves some cost? But ESP32 itself is more expensive than ESP8266. The only thing ESP32 would save is an IO expander (PCA9555) and thats not that expensive.