This is how I do it on my 3.8 site. It may need to be modified for 4.x. Also, remember that there is a cache of the feeds so you won't see the change immediately.
hook - external_query
PHP Code:
unset($vbulletin->GPC['fulldesc']);
$vbulletin->options['threadpreview'] = 150;
$hook_query_fields .= ",LEFT(post.pagetext,300) AS message";
If you get this to work on your vB4 forum, please let us know if the above code works or post the modified code.