It isn't the same as the 38xx. There's more than double the number of shaders, they've tweaked the ROPs, more than doubled the number of texture units, optimized the memory controller, and more. There's a reason the 4850 can outrun the 3870 easily, and the 3870x2 more than half the time, making the 8800 GT look slow by comparison. . .
My guess is there'll probably be some sort of low-end version for the Mac, but no 4870. Kind of like how they had the 2600 XT but never the 2900 in the iMacs.
It's too bad. I don't really even get why they bothered with the 3870 when the 4870 is over twice as fast.... but they did.