I'm not entirely clear on how the pairing for crossfire works. Can you pair any 2 cards that support crossfire or do they at least have to be closely related like if I have a 4870 would I have to use another 4800 series card?
I should further stress the fact that it should be the same model (4870 in your case), you could crossfire a 4850 but the way crossfire works is that your 4870 will downclock to the speeds of that 4850, that is why the dots are recommending the exact same model of card in that chart.
The manufacturer can be different, a XFX with a Sapphire or an HIS, it dose not matter.
Another thing to look out for is that if your 4870 is a 512 MB version its better paired with another 512MB rather than the 1GB version for the same reasons you would not crossfire with a 4850, the extra VRAM would be unused.