$(function(){
   $('#mainNav a')
      .css( {backgroundPosition: "0 0"} )
      .mouseover(function(){
         $(this).stop().animate({backgroundPosition:"(0 -4px)"}, {duration:250})
      })
      .mouseout(function(){
         $(this).stop().animate({backgroundPosition:"(0 0)"}, {duration:250, complete:function(){
            $(this).css({backgroundPosition: "0 0"})
         }})
      })
});