If you want to use different meta tags for different pages I recommend you to use custom field values.
If you prefer to do it with conditional functions, try this:
add_action( 'wp_head','carlos_head_meta_page' );
function carlos_head_meta_page() {
if ( is_page('4') ) {
$description = 'tag1';
} else {
$description = 'tag2';
}
?>
<meta name="description" content="<?php echo $description ?>" />
<?php
}