At the end of Fringe's third season, there was a significant change that you might not want to know about if you haven't viewed that far, so this is some introductory text to give you a chance to avoid reading any further. This question is composed of spoilers (for season 3 and earlier), so stop now if you wish.
The change was that the past was altered to a new timeline. As such, season four has some differences in the past from the history we'd seen played out during the first 3 seasons. One of the differences was that in the new timeline, the Fringe Division members Over Here don't know about Observers. Shouldn't this be impossible? Observers were first noticed when Walternate was working on a cure for Peter's illness. He stumbles upon the cure, but fails to realize it, as an Observer distracts him at a crucial moment. As a result, he moves on to another attempt at a cure and never finds one. This entire sequence of events was originally witnessed by Walter via his viewing device that let him see events Over There.
This prompts Walter to open a portal to Over There and bring Peter back. As such, the Observer's interaction with the moment Walternate found the cure is kind of crucial for the events of the show to have played out in both timelines. So how could the Observers still be unknown to people Over Here when we know that Walter went Over There for Peter?