WP_Query() load selected post

You don’t need a query if you the know the post ID:

<?php

$post_id = absint( $_POST['post_id'] );

if ( ! $post = get_post( $post_id ) )
    exit; // Error

if ( $post->post_status !== 'publish' )
    exit; // Might want to prevent script kiddies from accessing private content

echo '<h4>' . get_the_title( $post ) . '</h4>';