Check that image out. What you are looking at are "display stickers" that T-Mobile stores receive for in-store promotions. And those stickers above clearly show Windows Phone 7 and 4G.
But perhaps it's a mistake? Ah, but s second image shows just the regular Windows Phone logo. The 4G one seems in contrast to the regular one, strongly suggesting that this isn't a mistake and indeed, T-Mo are prepping for a brand new, 4G Mango phone. Perhaps that HTC Bresson we heard about awhile back?
Exciting times, folks.
Source: T-Mo News; Thanks, Gerry!