<div id="slider">
<?php
$tmp = $wp_query;
$wp_query = new WP_Query('posts_per_page=4&category_name=destaque');
if(have_posts()) :
    while(have_posts()) :
        the_post();
?>
            <a href="<?php the_permalink(); ?>"><?php the_post_thumbnail('vitrine'); ?></a>
<?php
    endwhile;
endif;
$wp_query = $tmp;
?>
</div>

In functions.php

if ( function_exists( 'add_theme_support' ) ) {
  add_theme_support( 'post-thumbnails' );
  add_image_size( 'homethumb', 260, 304, true );
  add_image_size( 'vitrine', 620, 280, true );
}