Defining multiple categories for showing latest posts

Question

I have a function that lets me display the latest posts on my website, however i want to function to show posts of multiple categories at not just one specific category. Check the code below and what i'm saying may start to make sense.

function latestposts()   {
    $args = array( 'posts_per_page' => 4, 'cat' => '4', '5', '6', '7');                  
    $last_5_posts_query = new WP_Query( $args );
    while($last_5_posts_query->have_posts()) : 
        $last_5_posts_query->the_post();
        $link = get_permalink();
        $title = get_the_title();                           

        $content .= '<div class="thepost">';
        $content .= '<div class="thumbnail">' 
        . get_the_post_thumbnail( null, "home-post-thumbnail") 
        . '</div>';
        $content .= '<h3><a href='.$link.' target="_top">'.$title.'</a></h3>';
        $content .= '<p class="excerpt">' .get_the_excerpt(). '</p>';
        $content .= '</div>';
    endwhile;

return $content;

I made an attempt of making it show multiple categories with the below code, however it only shows items populating category 4. How do i make it display the posts from multiple categories? I thought the code below would work, but alas, does not.

$args = array( 'posts_per_page' => 4, 'cat' => '4', '5', '6', '7'); 

Answer 1

This is incorrect formatting of a PHP array:

'cat' => '4', '5', '6', '7'

Use category__in and pass an array of category IDs:

$args = array(
    'posts_per_page' => 4,
    'category__in' => array( 4, 5, 6, 7 )
);

See WP_Query for all valid parameters and how to format them correctly.