The Radeon HD 7670 was released as an OEM product which means they sold only to system builders (Dell, HP, etc.). The HD 7670 is really just a rebranded Radeon HD 6670, so it's not even a new card.
The HD 4890 is roughly equal to a HD 6790 in terms of raw power. However, the HD 4890 is a DirectX 10.1 card. The Radeon HD 5000 series were AMD's 1st generation DX 11 cards.