The Producers have said that they have decided to end the series after it's fourth season since they have gotten near the ending they had originally envisioned.
http://www.variety.com/article/VR1117966089.html?categoryid=1417&cs=1&p=0
I'm glad the series is ending on it's own terms. I hate it when series end with an eye towards a feature film or spin-off (see Buffyverse) or stay on the air by stretching the story out (see X-Files). I hope the finale is satisfying. I hope the same thing about Lost's finale three years from now.
http://www.variety.com/article/VR1117966089.html?categoryid=1417&cs=1&p=0
I'm glad the series is ending on it's own terms. I hate it when series end with an eye towards a feature film or spin-off (see Buffyverse) or stay on the air by stretching the story out (see X-Files). I hope the finale is satisfying. I hope the same thing about Lost's finale three years from now.