The only Iphone that is 4G capable is the Iphone5. Verizon has nothing to do with that. The other Iphones do not have the hardware for 4G.
Yes, as the previous posters have said, 4G has been out for the iPhone since last year(September, I think).
Unfortunately, just as with any other brand of phone, this does not mean that older versions of the phone will have 4G capability. You have to purchase the newer model to take advantage of the newer technology.