Is there a way to capture part of a screen in full screen mode? I am playing a game that's called Bookworm on the Mac, and it runs in full screen mode.

If I press ⇧ shift+⌘ cmd+4 to capture part of the screen, it won't work.  Usually, I can press space and click to capture the whole window, but that doesn't work, either.

⇧ shift+⌘ cmd+3 works to capture the whole screen, but I'd like to find out how to take a partial screenshot.
Just use commandshift4 as usual. It just won't display its color overlay for the selection frame. The crosshair cursor should still show up and the screenshot should still be of the selected area.