new WP_Query messes up pagination

Question

I've been looking around for a solution to displaying a separate amount of posts specifically for the archive pages, and thought I had came up with some working code, however I see it messes up pagination. Basically page 2 of the archive displays the same posts as the archive front page. Link: http://hvamgolfklubb.no/category/ukategorisert/

Code:

<?php 

            /* Sets the amount of posts to display for $querycat */
            $querycat = new WP_Query( 'posts_per_page=4' ); ?>

        <?php
        /* Start the Loop */
        while ( $querycat->have_posts() ) : $querycat->the_post(); /* Display $querycat amount of posts */

            /* Include the post format-specific template for the content. If you want to
             * this in a child theme then include a file called called content-___.php
             * (where ___ is the post format) and that will be used instead.
             */
            get_template_part( 'content', get_post_format() );

        endwhile;

        wpbeginner_numeric_posts_nav();
        ?>

Any help is appreciated. I know it's not the numeric pagination that messes up as it behaved like this before implementing that as well. I'm still quite new to PHP, but improving every day, so please do share a better solution if you know of any.

Answer 1

If I understand you correctly you just want to display a different amount of posts at archives, for this you don't need a custom secondary query at all. You can control it via pre_get_posts hook, either making use of posts_per_page or posts_per_archive_page parameter.

Example for pre_get_posts

function wpse188786_different_number_of_posts_for_archive_ppp( $query ) {
    if ( !is_admin() && $query->is_main_query() ) {
        if ( $query->is_archive ) {
            $query->set( 'posts_per_page', 4 );
        }
    }
}
add_action( 'pre_get_posts','wpse188786_different_number_of_posts_for_archive_ppp' );

Example for posts_per_archive_page

function wpse188786_different_number_of_posts_for_archive_ppap( $query ) {
    if ( !is_admin() && $query->is_main_query() ) {
        // no $query->is_archive check needed, because posts_per_archive_page
        // does override, when $query->is_archive or $query->is_search is true
        $query->set( 'posts_per_archive_page', 4 );
    }
}
add_action( 'pre_get_posts','wpse188786_different_number_of_posts_for_archive_ppap' );