The manuals have a cap of how much exp you can earn with the effect active. That's the part that says (up to 4000) in your screenshot there. It wears off after you've gotten 4000 experience as a result of the effect. It's similar to how the empress ring worked in FFXI if you're familiar with that item.

Basically you can get 4000 exp on top of what you would normally earn then it wears off.