Quest Log uses storages to count.
Without looking at the scripts here's what I assume should be happening.
Gain quest storage by some means. (Npc, using an item, walking on a tile, killing a monster, using a talkaction. Doesn't matter.)
Now that you have the storage of the Quest, it appears in quest log.
Quest log main quest will be updated as completed when the quest storage is updated again.
The counter inside the quest log will most likely be a separate storage, however, depending on how it was scripted, may be the exact same storage.
This storage value would be updated using a creaturescript when the monster dies.
When you've completed all 4000, it will no longer update, and then you will need to finish the quest, by whatever means you choose, and Quest Log will be able to show completed.
Now looking at above according to your issue.
The quest storage is either incorrect or not updating.
In the particular position your in it can be one of three things.
The counter in quest log has an incorrect storage value, or is simply not set-up to count anything.
The creaturescript has the wrong storage values.
The creaturescript does not exist, and therefore cannot update the storage value.
Good luck,
Xikini