If it is locked, you'll have to unlock your iPhone 4S and then bring it to Koodo. You can also get a BYOD 10% discount on your plan because you brought your own device to Koodo; therefore, you have no outstanding tab.
You can use an unlocked device with Koodo, but you may want to use the Koodo TAB to get a newer device. The TAB is there to give you a `ready to use` device. There are a lot of in store promotions for activations rather than getting a TAB later.
Get a phone at the start = Better deal
Get a phone when it breaks = Worse deal