get_template_part in for loop

Due to the setup of my template (and layout), I need to be able to place 4 different posts in 4 different divs.

For example, my structure would be like this

<div>Post 1</div>
<div>
    <div>Post 2</div>
    <div>
        <div>Post 3</div>
        <div>Post 4</div>
    </div>
</div>

But I’m having some trouble getting this to work, I use get_posts to get the 4 latest posts.

$posts = get_posts(array(
    'post_type' => 'post',
    'post_count' => 4
));  

And then I try to display my post

<?php setup_postdata($posts[0]); ?>
<?php get_template_part( 'template-parts/post-thumbnail' ); ?>
<?php wp_reset_postdata(); ?>

In template-parts/post-thumbnail.php I’m trying to display the title and permalink, but it’s always showing the title and link of the current page. Never of the actual post.

Your problem is that the variable passed to setup_postdata() must be the global $post variable, like this:

// Reference global $post variable.
global $post;

// Get posts.
$posts = get_posts(array(
    'post_type' => 'post',
    'post_count' => 4
));  

// Set global post variable to first post.
$post = $posts[0];

// Setup post data.
setup_postdata( $post );

// Output template part.
get_template_part( 'template-parts/post-thumbnail' );

// Reset post data.
wp_reset_postdata();

Now normal template functions, like the_post_thumbnail() inside the template part will reference the correct post.