The problem will be that only one of the four carburetors will have the throttle control. The others are all linked up with linkages....So they will pretty much be useless unless retrofitted with the appropriate parts..
Then you will have to plug/cap remove all of the connections from the carb you are keeping...
The CBR carbs shown on e-bay are only 33 mm...Too small. The 39 mm FCR would work well on the 640 for low-end emphasis (that is the carb for the 660 Rallye).
The XR Carb should work like a charm with re-jetting!