Don't rush it. It doesn't matter if you don't finish before S4. Nothing, especially not a new season, could ruin this.

Eh, I want to get it done before whatever will happen after the premiere... happens. And whenever I come to the end of a fic, or sense I'm getting near the end, I start writing faster. Kind of like a horse that knows when it's going back to the stable--it'll always move a bit more smartly without extra urging :D