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Here are three different methods:

Many other variations could be possible, but I stop here ;-)

Here are four different methods:

Many other variations could be possible, but I stop here ;-)

Note that I've never installed or used this theme before. These are just some ideas based on skimming through the theme's source code.

We could try to modify method #2 by increasing the quantity option, to 6 but display only 4 (just as an example):

add_filter( 'longform_get_featured_posts', function( $posts )
{
    shuffle( $posts );

    if( count( $posts ) > 4  )
        array_slice( $posts, 0, 4 );

    return $posts;
}, 11 );

We could try to increase the theme's quantity option (from the backend) to 6 but display only 4 (just as an example). We could then use a modification of method #2:

add_filter( 'longform_get_featured_posts', function( $posts )
{
    shuffle( $posts );

    if( count( $posts ) > 4  )
        $posts = array_slice( $posts, 0, 4 );

    return $posts;
}, 11 );

Method 4)

We could try to modify method #2 by increasing the quantity option, to 6 but display only 4 (just as an example):

add_filter( 'longform_get_featured_posts', function( $posts )
{
    shuffle( $posts );

    if( count( $posts ) > 4  )
        array_slice( $posts, 0, 4 );

    return $posts;
}, 11 );

To increase the number above 6, then max_posts need to be modified from this code part:

// Add support for featured content.
add_theme_support( 'featured-content', array(
    'featured_content_filter' => 'longform_get_featured_posts',
    'max_posts' => 6,
) );

Many other variations could be possible, but I stop here ;-)