How to change selected attribute on click

[3cubes]

Hi people,
 
I have an issue that I'm trying to solve. Need some advice to the right direction:
 
When creating combinations, you have a dropdwon-list on product page where customer can choose the combination from. Here within the product.tpl file I have created some extra button to get customers attention for a certain combination. Now I would like this combination beeing automatically selected and activated when user clicks on that button. I have inputted some javascript code within the product.tpl file which is also hiding some other elements when button is clicked. I just dont get it to work that a certain attribute option is selected on mouseclick. Someone may have some clue to solve this??
 

This is the select option list of the attributes within product page. For example I want on button click choose option "Large" and beeing activated within product page via onclick function with help of jquery/javascript within the template. How to achieve this?

<select id="group_4" class="attribute_select selectBox" onchange="findCombination();getProductAttribute();$('#wrapResetImages').show('slow');;" name="group_4" style="display: none;">
<option title="Small" selected="selected" value="21">Small</option>
<option title="Medium" value="41">Medium</option>
<option title="Large" value="40">Large</option>
<option title="Extralarge" value="25">Extralarge</option>

[3cubes]

Thank you very much but I have already solved it now. For anybody who would like to know this is the right code:

$('#group_4 option[value="40"]').prop('selected', true).closest('select').trigger('change');

[Martin C]

I know it was a long time ago but can you please tell me which files you have changed?