Its DEFINITELY going to come out during the Christmas holidays season.
They cant afford to delay it anymore.
Other MMOs are slowly sucking away WOW customers and Blizz already acknowledged the fact that most players are getting bored with the current content.
Screw completion, they can patch that easily
S4 didnt come out together with 2.4 or AOC because Blizz wants a "respectable" amount of servers to progress far enough in Sunwell Plateau. (check the Blue post news)
That reason is quite obvious. It created anticipation among players long enough before the expansion. Release too early and people will get bored of the content too fast.
Besides that, they dont want to see players running around with S4 gear when most server barely even killed 1st 2 bosses of SWp.

Lol, goodluck with the beta key.
Its only available from the WWI event. You are NOT guaranteed to get invited for Wrath beta even if you have the key.
The beta key is random. Some rumors say that it will work for SC2 beta if you are not invited for Wrath.

Actually goblin is right

That's the main reason they pushed forward WOTLK

WTF? Runescape still exists?

So I'll be this lv1 zealot running around punching space rabbits for exp.

Me: Hi! Where is the nearest Hydralisk trainer? I need to learn Improve Spit 3
Zerg Guard: Take a turn to the left after you pass the Overmind quarters. I'll mark it in your minimap.