Im by far an expert. Just my thoughts here... Unless you do the whole damn i think you will waste your money doing the concrete. I would think repairing it would be the better way to go. I have never dealt with any type of sink hole or leak. So take my thought for what they cost ya. 4 inches over night is pretty big for sure. The problems lies way before just the surface hole. Since you say its not leaking out the damn its going into the earth. It could be a humugous hole that you dont see at this time. Might be a good idea to have the pros look into it.

The people who say I can't do it can just sit the @^#% down and watch me. Friends call me Rusto I also subscribe to pond boss mag. http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=504716#Post504716