Waldorf, I think he meant "I've now been waiting 4 days," not, "Apparently it takes 4 days to get it."
I don't know why it would take this long. Check your spam folder? I'd email Email. Are you sure you bought Galactic Civilizations, rather than accidentally pre-ordering Galactic Civilizations II?
As noted by PJ above, try contacting the sales department.
Generally if you buy a stardock product online you will receive the serial key via email immediately. So assumably it was caught by a spam filter or failed to send for some reason.