ATI has to get a little better with their pricing now. The 4670 put NVIDIA into sticker shock at $79.99 sending the 9600GSO in a freefall (hell, I couldn't resist picking one up at <$35). The 4670 is holding firm in price though, it needs to come down to at least $59.99 now with NVIDIA prices tumbling. If the 4830 is any more than maybe $99.99-$109.99 tops, and it performs nearly as good as a 4850, it will total cannabalize sales of the 4850 (which may be intended, that way ATI can use the full working 4850 yields to populate the 4870 market maybe? Bringing the cost for that card down a bit...). If the performance of the 4830 is closer to the 4670 than the 4850, then the 4830 will simply be an overpriced underperforming card making no sense against the 4850 when the 4850 can be had for about $140.
Basically, the only way I see this happening is if at launch of the 4830 ATI changes pricing:
4650 = $49.99
4670 = $59.99
4830 = $99.99
4850 = $149.99 (pre-rebate)
Note: The 4650 should not even exist at this point. It's a pointless card, unless ATI plans on using it at the extremely low end and forgetting about the 43xx series.