You can create SKUS for Product that don't usually have a barcode such as Deli or Menu items.
A 12 digit SKU that starts with 4 is an entire block of barcodes that is reserved for a store to use internally - manufacturers are not allowed to use them on products for public sale (outside of the chain).
I recommend the following way to make custom barcodes for your store - it is the official way to do it and will work on any POS system in the world:
The BackOffice can do it for you. All you need to do is Create a new product, and press "Generate Barcode"
If you followed the above instructions, your barcode when printed will scan on any POS system.