Hi James,
I solved it with a little tweak to your code, thanks for getting me pretty much there!
<?php if( have_rows('factsheets') ): ?>
<div class="vc_row wpb_row vc_row-fluid dt-default" style="margin-top: 0px; margin-bottom: 0px; min-height: 0px;">
<?php while( have_rows('factsheets') ): the_row();
// vars
$factsheet_title = get_sub_field('factsheet_title');
$factsheet_pdf = get_sub_field('factsheet_pdf');
?>
<div class="wpb_column vc_column_container vc_col-sm-3">
<div class="vc_column-inner ">
<div class="wpb_wrapper">
<a href="<?php echo $factsheet_pdf['url']; ?>"><img src="/pdf-logo.png" /></a>
<span><?php echo $factsheet_title; ?></span>
</div>
</div>
</div>
<?php
if( get_row_index() % 4 == 0 ){
// Close the div and open it again
echo '</div><div class="vc_row wpb_row vc_row-fluid dt-default" style="margin-top: 0px; margin-bottom: 0px; min-height: 0px;">';
}
?>
<?php endwhile; ?>
</div>
<?php endif; ?>
