previous_post_link inside of a function?

previous_post_link() and next_post_link() should work inside the the_content filter.

There is nothing wrong with your code except you must return the value inside the filter instead of print. Otherwise I believe post link should be there at the beginning of content.

previous_post_link() and next_post_link() print the output so consider using get_previous_post_link() and get_next_post_link().

function prevnext( $content ) {
    $content .= '<div>' . get_previous_post_link() . '-' . get_next_post_link() . '</div>';
    return $content;
}
add_action( 'the_content', 'prevnext', 4 );