Episode 19, "Letters of Transit," was an awesome episode, and it felt like watching a backdoor pilot for a new show. We got the future, evil Observers, Desmond from Lost, and a hot new agent...
...but it wasn't the same without Olivia though.
With only 2 more episodes to go before the season finale, reports have confirmed that the producers have filmed 2 endings: one if the series gets renewed for a 5th season, and another as the series ender.