T-Mobile, at least when this phone was released, did not have significant LTE coverage, but did have HSPA+ (21-42 Mbps download speeds). They advertised HSPA+ as "4G"
They probably weren't the only ones that "fudged" on that. It is definitely faster than 3G speeds for HSPA, but not all consider HSPA+ a "true" 4G.
T-Mobile ROMs and those "cooked" from them, not surprisingly, promoted the HSPA+ connection as "4G" in the UI. AOSP, CyanogenMod, and similar ROMs were a little more "honest" about things. There are patches somewhere to change the icon to "4G" if it makes you smile more.