When will WIFI calling be available for my S4 and my iPhone 4S
Your current devices are not compatible with carrier wifi calling, you'd need to get devices that are capable of using the advanced calling features, for example on Samsung devices it is the Galaxy S5 & newer, for Apple it's the model 6 & newer.
There are also 3rd party apps you can download for free to talk and text over wifi, however you'd be using a different phone number assigned by the app to do so.
