You can use for loop. See sample code below:
<?php
for( $i=1; $i<=4; $i++){
$prop_det_url = get_field('link'.$i);
if( $prop_det_url != '' ){ ?>
<a href="<?php echo $prop_det_url; ?>" target="_blank">> <?php the_field('link_titul'.$i); ?></a>
<?php }
}
?>