The Nexus 4 was announced just recently and one such carrier it'll work on is T-Mobile. The carrier has been touting a new feature called WiFi calling which is pretty straightforward and will allow you to make phone calls through your wireless Internet connection in case your coverage is a bit spotty.
It seems that customers were complaining of spotty coverage at home and they worked on a solution. While WiFi calling is available on a number of different devices, leaked documents showed that the Nexus 4 will not be getting it. It doesn't make a whole lot of sense.
Then again, now it's being said that a support document will be included with the Nexus 4 on how to setup WiFi calling. Maybe T-Mobile has had a change of heart, or they're ready to confuse a lot of customers. Either way, if you're wanting the Nexus 4, simply buying it from the Play Store for $100 more and unlocked might be a better bet.
If T-Mobile doesn't include WiFi calling, there's a great alternative called GrooveIP which uses Google Voice to make and receive calls from your phone. The app is both free (with ads) or you can completely remove the ads and get more functionality for just $4.99.