Display posts and thumbnails with certain tags

See the WP docs for query_posts.

Relevant excerpt:

The following returns all posts that belong to category 1 and are tagged "apples"

    query_posts( 'cat=1&tag=apples' );

You can search for several tags using +

    query_posts( 'cat=1&tag=apples+apples' );

Or using the array version you’re using, something like this:

$args = array(
    'category__in' => array(4),
    'posts_per_page' => 4,
    'tag' => array('Bolivia', 'Brazil')
)
query_posts($args);

String version for your case:

query_posts('category__in=4&posts_per_page=4&tag=Bolivia,Brazil');