They're not jumping from 4 to 7. 5 has been released; 6 will be released soon; 7 will be released six weeks after that. It's a time-based schedule with a new version number every six weeks.

Mozilla was sick of defining features for a given version and then delaying releases for months because of too-ambitious goals, so they switched to a whatever-is-done-by-the-deadline system. Version numbers aren't particularly meaningful.