<?php
$the_query = new WP_Query(
array(
    'post_type' => 'sponsors',
    'posts_per_page' => 4
)
);

if ( $the_query->have_posts() ) {
    while ( $the_query->have_posts() ) {
        $the_query->the_post();

        $counter = 0;
        echo '<section class="sponsor-row">';

            if ( has_post_thumbnail() ) {
                $counter++;

                echo '<div class="grid half">';
                    echo '<a class="sponsor" href="'.get_the_permalink().'" target="_blank">';
                    the_post_thumbnail( 'full' );
                    echo '</a>';
                echo '</div>';

                if ($counter <= 2){
                    echo '</section>';
                }
            }
        }
    }
wp_reset_postdata();
?>