As described in the web frontend:
[...] Accounts - Balances - All Brokerage Accounts'. Kontonummeret ditt står oppført ved siden av 'Individual' i fire + fire format
There, the complete account number is no longer shown.
The complete individual account number is shown under Profile -> Account Settings

There is also possibly an account number connected with the Equity Awards account under Accounts -> Statements & Tax Forms -> Employee Sponsored Accounts Tax Forms -> 1042S - YYYY -> form field 13k.