Updated code

<div id="slides">
                <div class="slides_container">

                        <?php $loop = new WP_Query(array('post_type' => 'fastighet', 'posts_per_page' => -1 )); ?>
                        <?php $i = 0; while ( $loop->have_posts() ) : $loop->the_post(); ?>
                        <?php if($i%4 == 0): ?>

                        <div class="slide">
                        <?php endif; ?>
                        <div class="fastighet-slider-entry">
                        <a href="<?php the_permalink() ?>" rel="bookmark" title="< ?php the_title_attribute(); ?>"><?php the_post_thumbnail(); ?></a>
                         <div class="caption">
                            <h5><?php the_title(); ?></h5>
                         </div>
                        </div> <!-- end .fastighet-slider-entry -->
                         <?php if($i%4 == 0): ?></div><?php endif; ?>

                        <?php $i++; endwhile; ?>
                        <?php wp_reset_query(); ?>

                </div> <!-- end .slides_container -->

                <a href="#" class="prev">prev</a>
                <a href="#" class="next">next</a>

        </div> <!-- end .slides -->