Hi Alex,
I ran into your question while looking for a different solution. I know it’s been 4 months, and you probably came up with a solution already, but I’ll attempt to answer this question anyways.
Based on your message, it looks like you were trying to use meta boxes to handle the forth image on your presentation layer or your theme. You could probably solve this by not using meta boxes and using a counter in your loop to catch for forth image with a PHP MOD(%). This way everything stays on your presentation layer.
For example:
<?php
$counter=1;
while ( $loop->have_posts() ) : $loop->the_post();
if ($counter % 4=0) {
echo 'forth image: so display it how u like.';
} else {
echo 'not forth image...';
}
$counter++;
endwhile;
?>