Well, there is no way a pool store can differentiate a CYA reading of 4. It is just not that precise and it also usually cannot detect levels lower than 30. So, we'll assume basically 0. How much CYA did you add and how did you add it? If you have some trichlor pucks in the skimmer, make sure you keep your pump running 24/7 while they are in there.

Go ahead and get some bleach in there. A pool with no chlorine can turn green FAST!