Tried very simple math. <div class="slide">
will print after every 4 post. So, if statement with old school logic $i%4 == 0
<div class="slides_container">
<?php
$args = array(
'post_type' => 'fastighet',
'numberposts' => -1,
'orderby' => 'ASC'
);
$posts = get_posts($args);
?>
<?php $i = 0; foreach($posts as $post): ?>
<?php if($i%4 == 0): ?>
<div class="slide">
<?php endif;
$url = get_post_meta($post->ID, "url", true);
if($url) {
echo '<a href="'.$url.'">';
echo the_post_thumbnail('admin-list-thumb');
echo '</a>';
} else {
echo the_post_thumbnail('admin-list-thumb');
} ?>
<div class="caption">
<h5><?php the_title(); ?></h5>
</div>
<?php if($i%4 == 3): ?></div> <!--end .slide--><?php endif; ?>
<?php $i++; endforeach; ?>
</div> <!-- end .slides_container -->
Let me know how it goes. Thanks!