<script src="http://code.jquery.com/jquery-latest.js"></script>    
<script type="text/javascript">

 $(function() {
		$("#blabla4").attr({value: "BРРРРР"});
 });

</script>
<form action="script.php" method="post">
	<table>
		<tr>
			<td>
				<input type=hidden name='blabla' value='blabla'><input type=hidden name='blabla1' value='get'>
				<input type=hidden name='blabla2' value='blabla3'><input type='text' id="blabla4" name='blabla4' value='A'>
			</td>
			<td>
			<input type=submit value='OK>
			</tr>
	</table>
</form>