I think COD4 is too new to start thinking about a COD5. Also, there is always a point where enough is enough. The last game they could make would be in the future (if they follow the pattern of their other games) and the whole excitement of COD is that the weapons really exist.
i think it would be cool to have a COD in the future but i think its a bit out of there range cos everyone thought that COD4 was a bit drastic in the time scale away from WW2