Appending a field in the categories array

Hi folks,

This must be basic PHP thing but my PHP knowledge is even poorer than that. I am trying to have 4 thumbnails (featured images) of random posts in the same category so I used this:

<?php $posts = get_posts('orderby=rand&numberposts=4&cat=category_id'); foreach($posts as $post) { ?>
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_post_thumbnail(); ?></a>
<?php } ?>

However my problem is that every post is in two categories and I need the code above to ignore second category.

For this piece of code

<?php $categories = get_the_category($post->ID);
var_dump($categories);

I get the following:

array (size=2)
  0 =>
    object(stdClass)[211]
      public 'term_id' => &string '38' (length=2)
      public 'name' => &string 'Advertising' (length=11)
      public 'slug' => &string 'advertising-2' (length=13)
      public 'term_group' => string '0' (length=1)
      public 'term_taxonomy_id' => string '43' (length=2)
      public 'taxonomy' => string 'category' (length=8)
      public 'description' => &string '' (length=0)
      public 'parent' => &string '0' (length=1)
      public 'count' => &string '2' (length=1)
      public 'object_id' => string '248' (length=3)
      public 'cat_ID' => &string '38' (length=2)
      public 'category_count' => &string '2' (length=1)
      public 'category_description' => &string '' (length=0)
      public 'cat_name' => &string 'Advertising' (length=11)
      public 'category_nicename' => &string 'advertising-2' (length=13)
      public 'category_parent' => &string '0' (length=1)
  1 =>
    object(stdClass)[212]
      public 'term_id' => &string '29' (length=2)
      public 'name' => &string 'All Logos' (length=9)
      public 'slug' => &string 'all' (length=3)
      public 'term_group' => string '0' (length=1)
      public 'term_taxonomy_id' => string '34' (length=2)
      public 'taxonomy' => string 'category' (length=8)
      public 'description' => &string '' (length=0)
      public 'parent' => &string '0' (length=1)
      public 'count' => &string '24' (length=2)
      public 'object_id' => string '248' (length=3)
      public 'cat_ID' => &string '29' (length=2)
      public 'category_count' => &string '24' (length=2)
      public 'category_description' => &string '' (length=0)
      public 'cat_name' => &string 'All Logos' (length=9)
      public 'category_nicename' => &string 'all' (length=3)
      public 'category_parent' => &string '0' (length=1)

How do I get cat_ID from the first object, please?