This is because you are not passing the parameters in the correct way, taxonomy param needs to be passed under `tax_query`. Please replace the `$args` with the below:--

    $args = array(
    	'posts_per_page' => 4,
    	'tax_query' => array(
    		array(
    			'taxonomy' => 'tag',
    			'field' => 'slug',
    			'terms' => 'pp'
    		)
    	)
    );

For more details check the **<a href="http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters">codex</a>**