Some of the facts this guy has put together are a little shaky but the B4 page on suzukicycles.org says '89.
I agree that the Ducati Monster and B400 look similar. I love the frame and think that Suzuki should have kept the look for the larger B600 and B1200. I've never investigated which came first - the Monster or B400 - but it looks as if one designer influenced the other.