<?php

    $categories = get_categories();
    $do_not_duplicate = array();
    $last_cat = array();

foreach ( $categories as $category ) {

   if($category->term_id!==4){
        $last_cat = array(
            'cat' => $category->term_id,
            'post_type' => 'post',
            'posts_per_page' => '1',
        //    'category__not_in' =>4,
            'post__not_in' => $do_not_duplicate
        );
   }//exclude

    $query_cat = new WP_Query($last_cat);

    if ( $query_cat->have_posts() ) {

        while ( $query_cat->have_posts() ) {

            $query_cat->the_post();
            $do_not_duplicate[] = $post->ID;

            $post_cat_thumbnail[] = get_the_post_thumbnail_url();
            $post_cat_title[] = get_the_title();
            $post_cat_excerpt[] = get_the_excerpt();
            $post_cat_url[] = get_the_permalink();

         }//while

    }//if

wp_reset_postdata();

}//foreach

?>