Four Square is a common playground game played mostly in grade school. Even though it is classically played in grade school, this game has been picked up and played in tournaments that have spread world wide.
With rising popularity, official rules and regulations have been made for the game in competitions. This includes the specifications for the ball (as listed below).
The ball of four square is an 8.5-inch rubber playground ball inflated to 2 lbs. They sell as “playground balls” or sometimes better known as “kickballs”. We are very fond of the Champion PG8.5 brand and model because of the two-ply rubber construction, grippy texture, and choice of colors. Many sporting supply companies that serve schools and leagues can arrange to have the balls custom printed with your own logo or design. To preserve the life of these balls we recommend storing them partially deflated when they won’t be in use for a long period of time (like winter).
You an improvise with any 8- to 10-inch diameter inflatable ball if you don’t have the luxury of investing in official balls. Soccer balls and volley balls are great substitutions. Our friends in Australia play four square with 3-inch felted tennis balls. If the ball bounces back to at least 50% of the height from which it was dropped then you’ll still be in the spirit of the game.
Rule – http://www.squarefour.org/gear