Butter baking pan; set aside.  Cook peanut butter, milk, water and sugar on medium-high heat stirring constantly.  Let boil 10 minutes.  Reduce heat to medium, stirring constantly for about 15 minutes.  When mixture reaches soft-ball stage (test in cup of cold water, mixture is soft and holds together), add butter.  Continue cooking and stirring for 5 minutes.  Test again for soft-ball stage.  Pour into butter baking pan.  Let cool until hardens.