Thank you both.
Meanwhile I got it to work perfect. The way was done with pre_get_posts.
My solution now is the function:
function numberposts_for_index( $query ) {
if( $query->is_main_query() && ! is_admin() && $query->is_home() ) {
$query->set( 'posts_per_page', '4' );
$query->set('post_status','future,publish');
}}
add_action( 'pre_get_posts', 'numberposts_for_index' );```
and my code in the index.php now:
```<?php
if(have_posts()) :
while(have_posts()) : the_post();?>
<?php the_content('<br />→ weiterlesen …'); ?>
<?php endwhile; ?>
<div><?php the_posts_pagination( array(
'mid_size' => 4,
'prev_text' => __( '←', 'neuere Beiträge' ),
'next_text' => __( '→', 'ältere Beiträge' ),
) ); ?></div><p> </p>
<?php else :?>
<h3><?php _e('404 Error: Not Found', ''); ?></h3>
<?php endif; ?>
<?php wp_reset_postdata();?>```