// WP_Query arguments
$args = array(
 'category__in' => array(1,3,4),
);

// The Query
$query = new WP_Query( $args );

// The Loop
if ( $query->have_posts() ) {
while ( $query->have_posts() ) {
$query->the_post();
// do something
}
} else {
// no posts found
}

// Restore original Post Data
wp_reset_postdata();