need custom loop code

you can use the build-in lop counter to determine the output of the li tags;
example:

<?php while( have_posts() ) : the_post();
if( $wp_query->current_post%4 == 0 ) echo '<li>'; ?>
<div>
<?php echo 'post'.($wp_query->current_post+1); //output here// ?>
</div>
<?php if( $wp_query->current_post%4 == 3 || $wp_query->current_post == $wp_query->post_count-1 ) echo '</li>';
endwhile; ?>

worked like a charm

Great help and thank you.

How to implement into this code with li tags?

That code works for posts

<?php /* Time to create a new loop that pulls out image attachments for this post */

$args = array( 'post_type' => 'attachment', 'posts_per_page' => -1, 'numberposts' => 999, 'post_status' => null, 'post_parent' => $post->ID, 'orderby' => 'menu_order ID', 'order' => 'ASC', );
$attachments = get_posts( $args );
if ( $attachments ) {
foreach ( $attachments as $attachment ) {
$image_attributes = wp_get_attachment_image_src( $attachment->ID, 'full' );

echo '<div class="centerImg">';
echo '<a rel="prettyPhoto-img" href="';
echo $image_attributes[0];
echo '">';
echo '<img src="';
echo $image_attributes[0];
echo '" />';
echo '</div>';

?>
<?php } ?>
<?php } ?>

you need to run a counter variable in the foreach loop, and check for ‘before position 1,5,9,etc’ and ‘after position 4,8,12,etc,or after last in loop’ – a pure php exercise and nothing particular to WordPress.

example:

<?php /* Time to create a new loop that pulls out image attachments for this post */

$args = array( 'post_type' => 'attachment', 'posts_per_page' => -1, 'numberposts' => 999, 'post_status' => null, 'post_parent' => $post->ID, 'orderby' => 'menu_order ID', 'order' => 'ASC', );
$attachments = get_posts( $args );
if ( $attachments ) {
	$counter = 0;
	echo '<ul>';
	foreach ( $attachments as $attachment ) {
		$image_attributes = wp_get_attachment_image_src( $attachment->ID, 'full' );

		if( $counter%4 == 0 ) echo '<li>'; //open <li> before every fourth item

		echo '<div class="centerImg">';
		echo '<a rel="prettyPhoto-img" href="';
		echo $image_attributes[0];
		echo '">';
		echo '<img src="';
		echo $image_attributes[0];
		echo '" />';
		echo '</a>';
		echo '</div>';

		if( $counter%4 == 3 || $counter == count( $attachments )-1 ) echo '</li>'; //close </li> after each block of four or after last item

		$counter++; ?>
	<?php }
	echo '</ul>'; ?>

<?php } ?>

WORKED!

THANK YOU AGAIN

Thank you so much! that helped me too ??