I just looked at the specifications of the products table and the identity field is limited to 4 numbers which mean you can only have maximum of 9,999 products in the system.
The four in the length field (column properties) for Nop_Product.ProductId means that the integer type uses four bytes for storing the value; it does not mean that it is limited to four numbers (characters).
MSDN: int, bigint, smallint, and tinyint (Transact-SQL)
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