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You can buy one with contract for $200, switch to dumbphone and then cancel the data. Therefore, you'd be using your iPhone as an iPod touch.

As a further addition, you can then trim your dumbphone sim and insert it into the iPhone 4, using it without data until ATT catches you.
 
When the network sees an iPhone IMEI online, it will add data to your plan.
+1

If you go the pay as you go route, AT&T can't add a data plan automatically.
However, you wouldn't be eligible for discount pricing on a new iphone; you'd have to pay full price.
 
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