One of the downsides to Google's Nexus sales model is that, if you want to get your hands on a Nexus 4, you have to order one. Thankfully, T-Mobile is also going to be carrying the newest flagship phone. If you want to save some money up front in exchange for a 2-year contract, you can pick up the device for $200. It's currently available online here, as well as in select stores.
Oddly, if you want to buy the phone off-contract from T-Mobile, it's going to run you a jaw-dropping $500. The handset is the exact same hardware that's being sold by Google, so there's no real reason to spend the extra $150 unless you really like T-Mo and feel like being generous. Of course, that assumes you can even get your hands on one right now. As of this writing, both the 8GB and 16GB models are still sold out on the Play Store.