In older versions of jquery you'll have to do it the "javascript way" using settimeout
setTimeout( function(){$('div').hide();} , 4000);
or
setTimeout( "$('div').hide();", 4000);
Recently with jquery 1.4 this solution has been added:
$("div").delay(4000).hide();
Of course replace "div" by the correct element using a valid jquery selector and call the function when the document is ready.