CC can accumulate, it depends on how much ammonia and urea or other organics are in the water. If you are killing algae then 0 ppm CC is a good indicator (along with the Fc holding overnight) that you have killed all the algae. If you do not raise the FC high enough to destroy all the orgainics in the water you will still have CC left.

Your example of putting in 10 ppm FC at 6 pm and losing 4 ppm overnight indicates that the chlorine is fighting something in the water and you need to continue to shock until the FC holds overnight when UV degredation of chlorine is taken out of the equation. This is a basic of the BBB method on here. Just read the posts!

"Shock" is really raising the FC high enough for the level of CYA in the water to insure that there is enough 'active' chlorine to do the job and it is not bound up with the stabilizer in the form of chlorinated isocyanurates but is in the form of hypochorous acid.

It is a combination of having no CC and having your FC levels hold that indicates that you do not have a chlorine demand in your pool. If this is not the case you need to continue shocking and possibly try shocking to a higher FC level. It is also important to keep the FC at shock level by adding more chlorine at least a few times a day.