Customise was moved to the Window>Workspace menu after the initial release (or at least the initial beta) of DS4.0, and menus don't get updated automatically so if you have a custom layout and keep suing it you will get the old location. It's annoying, but it is a good idea to go back to one of the standard layouts and then reconfigure it from time-to-time. (Actually, I think the recent versions are a bit smarter about updating menus, but still it's something that can be a gotcha.)