Switching from 3G to 4G phone

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http://androidforums.com – My wife has the iPhone 4S with unlimited data. I jumped into the smartphone world late in the game and never got unlimited data. Anyways, my question is, when my wife upgrades to a 4G phone, will she be able to keep unlimited data? When I went from a 3G phone to 4G, Verizon told me I had to get a 4G data plan. So I'm thinking my wife will have to do this as well and we'll lose her unlimited data. I hope not. (General)