How do I paste what I've captured using cmnd-shift-4?

That key combination takes a screen shot and prints the image to a file, not the clipboard. Just open the image file in your document, or open it in preview and copy and paste from there.

Or hold down one extra key with the other 3 - ctrl - and the screen shot goes to the clipboard, ready for a simple paste. It works for the full-screen capture (3) as well.

True! I forgot about that one.

This is a useful page to have handy for any OS X user (keyboard shortcuts listing)

http://support.apple.com/kb/HT1343