SELECT DISTINCT brand_id FROM your_table WHERE section_id IN (3,4)
section_id IN (3,4) is the same as section_id=3 OR section_id=4
SELECT DISTINCT means SELECT, and remove all duplicates.
So for this query without using DISTINCT you would get 13,12,13,12
With distinct you get 13,12
Of course you can add an ORDER BY brand_id to sort the results according to brand_id.
When using PHP you can do something like
$section_ids = array(3,4);
$result = mysql_query('SELECT DISTINCT brand_id FROM your_table WHERE section_id IN ('.implode(',', $section_ids).')');
Does that answer your question?