Radeon HD4850 cards are manufactured and we heard that a few hundred thousand will be available next week after the launch, but at the same time the future won't be so bright for Radeon HD 4870.
RV770XT-based Radeon HD 4870 is affected by shortages of GDDR5 memory and the cards are barely going to make it for the last week launch.
The good news is that Radeon HD 4870 should be available after some initial shortage,s but we were assured that the volumes of these cards will be shipped as of early July.
Radeon HD 4850 should be widely available, while 4870 will be out, but it might not be that easy to get one, at least in the first week or two.