When a show can get an early pickup it is always good. Rushed, unthought out story-arcs usually cause a show more filler episodes. Creator Eric Kripke discusses his plans for the coming season with SciFi Wire:

Gamble added that the early pickup by The CW means that the writers can set things up as the third season winds down. “We’re especially excited that we heard about the pickup early enough that we can start working on what we’re going to be doing at the beginning of next season now so we can thread some things in,” she said.

The producers are thinking in terms of a five-year arc for the series, Gamble added. “There are aspects of the mythology that have always been plotted out,” she said. “There’s approximately a five-year plan that’s been in place since the show began.”

Anytime a creator can have a series arc planned out from the beginning it makes a show better. Think “Lost” prior to the announcement of the end; disjointed and confused, now has a focus. “Prison Break” was another show with an set arc (three seasons), but they’ve abandoned that plan.

It’s not to say that a show can’t continue on past its plan, but you need to introduce a new arc in a season prior to the final, and wrap up the original arc. If “Supernatural” plans on continuing past season 5, hopefully the start to “thread some things in” to start that new arc.