Cmd-1..n are handy for changing and focusing the different navigator tabs so I don't have to take my fingers off the keyboard, but is there a shortcut for giving the focus back to the editor after focusing, say, the Project navigator with Cmd-1?
Aw crud, there are all sorts of "move focus to" commands in the Navigate menu that I somehow missed earlier... The combo for "Move Focus to Editor..." is Cmd-J, for example.
I don't have X-Code 4 installed, but Command + ~ will cycle through windows inside an application. So long as it qualifies as a window, I imagine that'd be what you need.