Use setup_postdata() to use template tags when using get_posts() to query the database

setup_postdata-and-get_posts.php

<?php
$args = array(
  'numberposts' = '4',
  'category_name' = 'slider'
);

$slider_posts = get_posts( $args );

if ( $slider_posts ) {
  foreach ( $slider_posts as $post ) :

    // First, setup post data
    setup_postdata( $post ); 

    // Second, define content to display ?>
    <h2><?php the_title(); ?></h2>
    <?php the_content(); ?>
    <!--setup_postdata() required for the_title() and the_content() to work--><?php
  endforeach;

  // Third, reset post data after end of the foreach function
  wp_reset_postdata();
}