run javascript with in_category

Hi there, I want to use in_category to run a little javascript in only one of my categories. I’m using this code just above the </head> tag:

<?php
 if ( have_posts() ) { the_post(); rewind_posts(); }
 if ( in_category(4) ) {
 echo "";
 }
 ?>

Once this works correctly I’ll put the javascript into the if statement. It currently puts the <br /> in category 4 but it’s also doing it on the index.php page. I’m assuming this is because the index.php page is currently displaying posts from category 4, but how can I stop this from happeneing?

Thanks.