Regardless of the electronics and circuitry used between the HD4850 and HD4870, they both use the same RV770 GPU core. So whatever temperatures Gpu-Z is polling, they are going to come from the same place for both cards. It's already been established what temperatures 1, 2, and 3 are (Display IO, Memory IO, and Shader Core). If you're seeing a large discrepancy between the temperatures, chances are very good that the VGA heatsink/waterblock isn't making proper level contact with the GPU core. This problem has been seen numerous times now, here on the TPU forums...
PS: I may be horribly wrong about everything I just said, but it's my educated opinion...