Two excellent games for four year olds are Hungry Hungry Hippos and Candy Land. Boths games are rated as appropriate for four year olds by their manufacturer, and have sold millions of units in the United States.
Hungry Hungry Hippos is a four player tabletop game in which all players use a plastic hippo to try to eat as many marbles as possible. In the process, children learn hand-eye coordination and counting skills.
Candy Land is a simple, four person board game where players advance along a path based on the cards they draw. Gameplay requires color recognition and basic counting, which are important skills for four year olds to learn.