Set up as for the three-player version. Then, before play begins remove tribes from the following locations:
In addition, when stealing a card, the player having fewer victory points may steal two cards rather than the usual one, if he wishes. He may decide after seeing the first card taken.
Otherwise play as usual.
Because of the four technology races, the race for chits and the reduction to just four types of raw materials, this version of Catan seems to feature less trading anyway. Players may continue to trade if they like, but it is expected that it's at best a gamble for at least one of them.
The change in the behavior of the Neanderthal and Sabertooth Tiger is to help prevent a "runaway leader" problem.