Loop by category ID

// WP_Query arguments
$args = array (
	'cat'                    => '4',
	'pagination'             => false,
	'posts_per_page'         => '5',
	'ignore_sticky_posts'    => true,
);

// 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();