The first time you launch iFly on a new device, it will ask you to log in so it can establish a connection with the update servers and download current data. The servers track which devices you have logged in from ("registered devices"), and if you try to log in from a fifth device, you will get an error message.
You can deregister a device from within iFly (any registered device can de-register itself or any of the others), to free up a spot for a new device.
So yes, you can do what you are asking to do.