We played 3 human players on map clover-3. one human player had a settlement on top left side of the top 9 field (the center one on the top leaf), one on botten right side of the lower left 9 field (the center one on the left leaf). After having built an additional road, no additional settlement, and having nothing even near a 3-1 port, he noticed that he could not trade 4-1 with the bank, but COULD trade 3-1 with the bank. this continued throughout the game for any 3-1 trade he tried, even though he never reached a 3-1 port. (There was only one 3-1 port anyway). The other players were only able to trade the usual 4-1 with bank.