I would guess the 4000, 5000 etc numbers are for the series of the card, then you have the speed/memory rating in the hundreds for example 6600 is series 6? With something of it running at 600 :s while the 6800 is the same series but runs at 800 something...
As you can see I have no clue as to what I'm saying so I think its best I shut up now!