How to display a specific category using a custom Query in WordPress?

Hi I want to display a specific category from a custom query in WordPress. My code works fine and it gets the latest 4 posts, but what I want now is to retrieve a specific category. My code is below thanks :

global $wpdb;

$posts = $wpdb->get_results(

    'SELECT ID, post_title AS title, post_excerpt AS excerpt
    FROM '.$wpdb->posts.' 
    WHERE post_type = "post" 
    AND post_status = "publish" 
    ORDER BY post_date DESC 
    LIMIT 4'

);