CTRL+Alt+4 works in CS, as long as the font that you are using has a € sign in the font file. Many fonts don't, and you'll usually find that with the ones that don't CTRL+ALT+4 will give you a question mark.
When you say 'a shortcut for something else' do you mean that it's a shortcut for a tool or something? If so then you need to make sure that you've selected the type tool, set it to you're document, and that the cursor is flashing ready. If it isn't then the CTRL+ALT+4 probably initiates some command (not sure what as I don't have photoshop on this computer to test it out).