Version 40 is the first to use a new numbering scheme. The previous version was 3.38, but the project did not want to get to 4.0 out of concern that it would be perceived as too big a shift. "If we ever did release 4, then people would see it as a huge change in [that] everything's going to be broken again, and that's not really what we've got,"
So instead of 3.38 to 3.40 or 3.38 to 4.0 they went from 3.38 to 40.