Right now there's no way to use wifi calling on the Nexus 4, although a lot of people are looking for it. I think there's a bounty posted somewhere if anyone can get it to work.
Has anyone figured this out? I talked to tmobile support and they made it sound like the only thing needed is compatability with the bands, and then installing the app may work..
As ryjb said, WiFi calling support requires modification of system files. Installing an app isn't sufficient. Currently there is no known way to use T-Mobile WiFi calling on a device that doesn't support it out of the box.