In English, Captivate doesn't have the hardware to pull off a 4G speed, the same way the oldest iPhone can't use 3G speeds. The best we can do is use HSUPA, which is faster than 3G and slower than 4G which the T-Mobile Vibrant is able to use (I think it's called 3.5G, but I can't confirm that). The Captivate is able to use it, too, but AT&T has only given access to HSUPA to the iPhone. Time goes on, blah blah, ROM's start to come out with HSUPA (check XDA's development section for the Captivate), but ROM's such as Cognition stopped integrating HSUPA into their ROM's since they claim that between Froyo's internet speed and HSUPA, there's not that big of a difference and that there is a lot of battery drain with HSUPA. Basically, they don't think it's worth it for the small speed increase, but a lot of ROM's still have it if you're interested.