<?php if( have_rows('your_repeater_field') ):
 $total = count(get_field('your_repeater_field'));
 $i = 1; ?>
    <div class="flex-wrap">
    <?php while( have_rows('your_repeater_field') ): the_row(); ?>
        Do whatever with the repeater item
    <?php if ( $i % 4 == 0 && $i != $total ) {
      echo '</div><div class="flex-wrap">';
    } ?>
    <?php $i++; endwhile; ?>
    </div>
<?php endif; ?>