Have you contacted the Rock Hill Parks & Recreation Dept? They start soccer at 5 years old (Kickers) and football isn't available until they are 7 years old.
RH Parks & Rec Sports (http://www.ci.rock-hill.sc.us/dynSubPage.aspx?deptID=13&pLinkID=104 - broken link)
York County has football but unfortunately the age is 5 years with minimum weight of 35 lbs.
York County Pop Warner
Some of the karate locations around town may take 4 year olds. You can call the YMCA and see if they have anything for your son's age group.
Good luck I hope you find something for him.