Based on the past expansion and I’m assuming that they did not change it up this time.
So doing a little theory crafting on my own I found that when 3.02 was released it was 30 days before the release of wrath of the lich king. Based on that model if patch 4.0 is released next Tuesday ( 21st of September ) I can see it released on October 21st which is a Thursday ( Like WotLK ). Let’s say the patch is not released next Tuesday but the week after ( 28th of September ). Then you can see it on Thursday,October 28th. A couple big wow sites/blogs say it will be on November 2nd, I just don’t think they will let us play 4.0 for that long.
This is all based on speculation and please don’t hate me if it is wrong , I don’t work at Blizzard so it really is impossible to tell you the exact release date.