Now as far as I can tell What Will Happen In Eragon IV? should be published in the first week of October - pretty much a month to the day after its original publication date. Now I've noticed that books usually seem to hit shelves either on Tuesdays or Thursdays for some reason so you can probably start looking for it in the US as early as Tuesday September 29th, but I suspect it won't be on the shelves until the following week - October 6th. As for Canada I'd say the earliest would be Thursday October 8th with Tuesday the 13th being the most likely.
It's ironic isn't it that one of the things I try to predict in the book is the publication date for Christopher's book, and here I am doing the very same thing for my own. Very strange don't you think? Funny old thing life isn't it? Anyway that means we're looking at the earliest a week - seven days - until the book is on shelves somewhere and three weeks - twenty-one days - at the latest.
In the mean time please feel free to leave your comments and predictions about what you think will happen in the series if you want. Just remember that you have to be able to give some pretty good reasons for your opinion and no slanging somebody else's opinion without backing it up with a good argument - "You're off your nut"- while fun to say and sometimes a valid editorial comment does not constitute a good argument under these circumstances.