The last of the 650 SOHC4's had non adjustable carb floats, if you can hear liquid in them you are screwed!
I think you will have to replace the lot, though how all 4 ended up with gas in them is a puzzle.
I had the older 79 CB650Z and a later custom 650, one float on the Z sank and I ended up buying a full set of old carbs for $50, swapped out the one float and never had any more trouble..
I seriously think you will run into trouble trying to replace the "T" pipes between the carbs as it is hard enough wrestling the 4 carbs in but if they continually spring a leak it will break your heart pulling them all the time.