Where did you find that info? They do not have the same speeds.
according to newegg, most 4850s and 4870s have the same amount of memory, although the 4870s have some 1GB cards.
The clock speeds are a bigger difference than I have origonally imagined.
Most of the 4850 and 4870 cards are rated at memory speeds of about 1800 MHz but I do see some that have ratings of 3600MHz.
They also have the same stream processors and phisical chip type on the cards.
Other than this, I don't really see any MAJOR differences between the the two cards.