This requires the player to have 4 different tradeable resources. If you play the Grand Campaign in co-op this'll come really quick. Both players are occupying a lot of cities and settlements, and are building new resource buildings. If you have a trade agreement with your partner, you'll get to 4 resources really quickly together.
This is also very easy to obtain in singleplayer though. Just remember to build the resource buildings in the settlements you obtain.