From what I can find reliably these are the ONLY supported GPU's-6450,6570 or 6670
So from what I see,No, the 4870 will NOT work.
Borrowed from guru3d user sykozis:
There have been a lot of people claiming to have success with the HD7750, but only 3 cards are officially supported. In reality, there's no point in trying "Dual Graphics" with an HD7770 as it won't provide any more benefit than an HD6670 will due to the considerably lower performance of the integrated GPU on the A8 6600K.
Plus, being 2 completely different architectures (the HD7700 series being a GCN based GPU and the Radeon HD8570D being a VLIW based architecture), even if you manage to get "Dual Graphics" to work, it's likely to be more problematic than using 2 VLIW based GPUs.
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