Can I get a 4G data plan for my unlocked iPhone 5 in the US?
T-Mobile's 3G/4G coverage for an iPhone 5 is better than a year ago but still limited so you are rolling the dice. They just released a carrier update for the iPhone that will give an iPhone 5 access to some of their 4G network. But it's still early days and the usefulness will greatly depend on your location. 

T-Mobile also offers a pay-as-you go plan where talk and text are charged as used and data can be activated on a day-to-day basis.

StraightTalk is another option for an unlocked iPhone but is only 3G. $45/month for unlimited talk, text, and data.  As usual, "unlimited" means "secret limits" that we can arbitrarily use to kick heavy users off.

The big advantage of T-Mobile is that you can walk into a T-Mobile store and get it set-up. AT&amp;T is still officially restricting the use of non-contract iPhones on their network and walking into a store will be a bust, but they don't actively hunt them down so GoPhone is an unofficial option.  You have to buy a GoPhone SIM and cut it down to size but it will work.  Unofficial and a bit of a PITA but the option exists. You can buy pre-cut cards on Amazon but getting one in a store is a bit trickier.

http://www.dargadgetz.com/how-to-use-iphone-5-on-att-gophoneprepaid-with-4g-data-speeds-payasyougo/

I suspect this gives 4G/HSPA+ and not 4G/LTE.