While you hold down Cmd+Shift+4, you can also hit the:

• Spacebar to drag the selected capture region around the screen.
• Shift key to vertically or horizontally lock the capture region.
• Option key to expand or reduce the size of the region proportionately.

To make things even a bit more confusing, you can combine some of these keystrokes. Shift and the Space Bar together will allow a fixed-size region to be dragged in either a vertical or horizontal direction, depending on which direction you first move the mouse after pressing the keys. If you combine Shift and Option, then you can grow your region from the center, restricting either the vertical or horizontal size. As I said, it’s a bit confusing, so here’s a movie of the options in action.

Screen Capture in Snow Leopard is a post from: My MacOS X