As BGR states, it is a bit late for Apple to be releasing an iPhone 4 on T-Mobile USA. It’s more likely that Apple is testing hardware to prepare for AT&T’s buyout of T-Mobile. If the merger does go through successfully without any issues, then dropping an iPhone that supports both AT&T and T-Mobile’s 3G would be very important.
Of course, Apple is always testing new stuff. It’d be foolish of them to ignore entire wireless networks. Even before the iPhone 4 landed on Verizon, it was discovered that Apple had been working with CDMA frequencies to test a possible CDMA iPhone. In the long run, an iPhone for every network in the U.S. is what Apple is shooting for.
While an iPhone 4 might not be coming out on T-Mobile USA just yet, it is a good sign of things to come. Perhaps they might get the iPhone 4S/5.