Apple tonight posted iPhone 4.0 beta 4 to the Developer Center with a strong clue that AT&T will finally enable data tethering. The new test release not only enables a "set up tethering" preference for American users but, when the menu is tapped, asks users to either call AT&T or to visit the carrier's website to turn the feature on. That it isn't automatically enabled suggests AT&T will charge an extra fee; for other smartphones, it normally charges an extra $30 per month.
