The rules for the 4-player game concern the addition of the Market Location to the game. During Setup, lay out the Market Location with the others (with one of each resource on it), and make sure that each player has their Gather Market card in addition to their usual three.
The Market is treated like any other Location. If multiple Pigs end up in the Market during the Gather phase, the resources there are divided by drafting. In turn order, those players take turns choosing 1 resource at a time. Each player will collect an equal number of resources, leaving any leftover resources behind. (For example, if there are three players and five resources, each player will get one, leaving two behind.)
During the Clean Up phase, place 1 of each resource (Straw, Wood, and Brick) into the Market, adding to any resources remaining from the previous round.