Like other Bluetooth fitness sensors, 4iiii powermeters only transmit on one Bluetooth channel. Once that channel is occupied (the powermeter is paired to a device), it cannot be paired it to another device.
In order to pair your powermeter (or Viiiiva Heart Rate monitor) to the Ride App, you must first ensure that it is not connected to any other device or app via Bluetooth, including the 4iiii App on your iPhone. If it is connected to the 4iiii App on iPhone, delete it from the app, and it will then be discoverable on the Ride Watch App.