I don't remember where I heard or read this, but I seem to recall there being some sort of cataclysmic event between 3.5 and 4e.
A few (related) questions -
- What exactly happened between 3.5 and 4e?
- How much do the current inhabitants of the Forgotten Realms know about what happened?
- How much game-world time has passed between editions?
- What geographic locations were affected by the events between editions, and how do they affect 4e play?