Storefront display four product categories on the homepage.

storefront-four-homepage-product-categories.php

/**
 * Alters the output of the homepage product categories on the Storefront theme
 * Affects the storefront_product_categories_args filter in /inc/structure/template-tags.php
 */

function sd_display_four_home_product_categories( $args ) {

	// Sets the maximum product categories to 4, you can increase this to display more if need be.
	$args['limit'] = 4;
	
	// Displays the number of columns as 4 instead of the default 3.
	$args['columns'] = 4;

	// Output
	return $args;

}
add_filter( 'storefront_product_categories_args', 'sd_display_four_home_product_categories' );