Is it just that simple to make a fourth resource???
Such a change would need a place to display the new resource on the main screen, and a variable to store the amounts of each resource for each player (which might be taken care of internally by the game), and it just came up as a stray thought...
I could make extractors, and declare them of a new type, where it inputs resource type, but I haven't tried it to see if a minidump occurs.
I'm mostly suggesting it to get wheels spinning on evolving the game past it's current level of sophistication, because I would SO VERY MUCH like to emulate "Civilization" with this engine!!!