Generation /Family/ Variant there in.
So a 4870 is part of the 4 series or generation as they like to call it. The 8 denotes the family, so 48XX are all the same Generation and family, and the last two numbers are the variant which is like the old Pro/LE/XT etc monikers. So you have a 4830/4850/4870/4890 Which to me would be 4800LE 4800PRO 4800XT and 4800XTX in old numbers.
Higher is better is not true, Within a generation or family its right as far as i can remember, cant think of one that isnt. However the 4770 is a better card than the 4830 so higher is better isnt a very good way of looking at it.
Mactronix