Can the Apple Watch app be side loaded and used on other iOS devices?
Apple Watch uses Bluetooth to communicate with iPhone. iPhone 5 is the first device that had Bluetooth LE support, which is used in cases where you want a low-energy-usage always-on connection, so it would follow that is the earliest model that can actually communicate with Apple Watch.

While Apple Watch can, in some circumstances, use Wi-Fi to communicate with iPhone, it has to be a network that iPhone was connected to while previously paired with Apple Watch over Bluetooth. Since devices prior to iPhone 5 likely cannot establish a Bluetooth connection to Apple Watch, no Wi-Fi network would ever be trusted by Apple Watch to use for communication with iPhone.