Yes, Mordecai Richler has left this Earth, but still a fourth Jacob Two-Two book is forthcoming. Tundra Books has contracted Cary Fagan to write a fourth book in the Jacob Two-Two series. You can read about it here.
I enjoyed Fagan's Kaspar Snit books a great deal, so I look forward to seeing what he does with Jacob and friends. (He sends him to sea, that much we know.) Fagan has the appropriate level of zaniness in his writing that's for sure (and, as I recall, satire, which is also Richler's schtick). So, I'm excited about how this will turn out. What I'm very excited about though, is that Dusan Petricic has been contracted to illustrate all four of the books. I adore his fabulous style. I know what's going on my 2009 Christmas Wish List!