How to display last 4 posts of cat 2,3 &7 (for ex.)?

I know how to display last posts for one specific cat, using:

<?php

// The Query
query_posts( 'cat=3' );

// The Loop
while (have_posts()) : the_post(); ?>

<h1><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a></h1>
<h2><?php the_time('l j F Y') ?></h2>
<?php the_content('Lire la suite &raquo;'); ?>

<?php endwhile;

// Reset Query
wp_reset_query();

?>

But I did not find the way to display last x (4 for ex.) posts from different categories (for ex cat 2, 3 & 7). I want those posts to be displayed from the most recent one to the oldest one (of the 4).

I read this page but a bit messy for the beginner I am:
http://codex.wordpress.org/Template_Tags/query_posts

Any help would be higly appreciated!