You can use custom post type for special posts on your blog and there are lots of plugins or hacks available to display related posts but those are limited to the general posts only, not for the custom post type.
Say you have created a new post type function for listing your products on your WordPress website or blog. On the product detailed page (single.php) you just prefer to display the other products info which belongs to the same product category. Now the trick will work.
Assume you have created (registered) a new taxonomy for your product categories as ‘product_categories’from your functions.php file. Now you can use the template code to display the related products (posts) in any place of the page you prefer. It will display latest 4 posts of the same category, no duplicate at all.