Here's where it gets tricky. LTE Advanced and WiMax 2 are considered 4G (they don't even exist yet). As of yesterday, ITU (the ones who decide what "4G" is) admitted that WiMax and LTE can be called 4G. T-Mobile uses HSPA+, which is basically really fast 3G. However, even as an Evo owner (Sprint uses WiMax), I have to say I'd rather have LTE or at least HSPA+. Why? Because it's always on as long as you're in an area that has it. And on top of that, it doesn't rape your battery like WiMax does. Besides, you shouldn't by a phone based on how fast the internet is. By the phone for its features, build quality, etc. It's faster than what you already have and that's what matters. Plus, you won't be paying extra for the extra speed.
http://www.engadget.com/2010/12/18/itu-capitulates-admits-that-the-term-4g-could-apply-to-lte-w/