Example One
OR
<?php if(!is_paged()) { ?>
<?php
$args = array( 'post_type' => 'slider', 'posts_per_page' => 4 );
$loop = new WP_Query( $args );
?>
<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<li>
<?php the_post_thumbnail('slider-image', array('class' => 'postthumbnails')); ?>
</li>
<?php endwhile; ?>
<?php wp_reset_query(); ?>
<?php } ?>
Example Two
<ul class="slides">
<?php
global $post;
$args = array( 'posts_per_page' => 4, 'post_type'=> 'slider-items');
$myposts = get_posts( $args );
foreach( $myposts as $post ) : setup_postdata($post); ?>
<?php $slider_image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'slider-thumb' ); ?>
<li data-thumb="<?php echo $slider_image[0]; ?>">
<?php the_post_thumbnail('slider-larger'); ?>
<?php the_content(); ?>
</li>
<?php endforeach; ?>
</ul>
<?php if(!is_paged()) { ?>
<?php
$args = array( 'post_type' => 'slider-items', 'posts_per_page' => 6 );
$loop = new WP_Query( $args );
?>
<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<div class="pc_item">
<?php the_post_thumbnail('slider-image'); ?>
</div>
<?php endwhile; ?>
<?php wp_reset_query(); ?>
<?php } ?>
OR
<?php if(!is_paged()) { ?>
<?php
$args = array( 'post_type' => 'slider', 'posts_per_page' => 4 );
$loop = new WP_Query( $args );
?>
<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<li>
<?php the_post_thumbnail('slider-image', array('class' => 'postthumbnails')); ?>
</li>
<?php endwhile; ?>
<?php wp_reset_query(); ?>
<?php } ?>
Example Two
<ul class="slides">
<?php
global $post;
$args = array( 'posts_per_page' => 4, 'post_type'=> 'slider-items');
$myposts = get_posts( $args );
foreach( $myposts as $post ) : setup_postdata($post); ?>
<?php $slider_image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'slider-thumb' ); ?>
<li data-thumb="<?php echo $slider_image[0]; ?>">
<?php the_post_thumbnail('slider-larger'); ?>
<?php the_content(); ?>
</li>
<?php endforeach; ?>
</ul>