The next T-Mobile myTouch, which looks to be a fine phone, as T-Mobile's first video-chatting Android phone, is being re-branded as the myTouch 4G. But it's not really 4G. It's pretty lame of them to call it that.
The myTouch, along with the G2, are T-Mobile's first phones to run with the HSPA+ networking standard. While you can jump headfirst into the technical details if you'd like, simply put, HSPA+ is 3G. It's very fast 3G, but it's technically a 3G technology.
T-Mobile's using phrasing like "4G speeds" and 4G branding because right now—as our own testing has shown—where there's HSPA+ coverage, you can get download speeds that are definitely on par with WiMax, and that match the lower end of what Verizon's initially promising for its 4G LTE network.
(Update: Colloquially, most people would consider WiMax and the first version of LTE being rolled out by Verizon and AT&T to be 4G, but neither actually meets International Mobile Telecommunications Advanced specs for 4G—LTE won't meet those until the second version, LTE Advanced. Similarly, 802.16m (read: WiMax release 2) will be the version of the tech to actually meet the IMT-Advanced specs.)
T-Mobile's prudent for pushing ahead with HSPA+. It's much cheaper and faster to upgrade existing networks to HSPA+ than rolling out brand spanking new 4G networks (as you can see, since they're already launching it, while Verizon's only going to start launching LTE this year and AT&T won't jump on it until next year). And the real world speeds, for now, are pretty comparable to what you'd see on existing "4G" networks. But it really shouldn't be trying to actively confuse consumers. Tech is tricky enough as it is without willful misdirection. [T-Mobile]