Definition
An NR4 is issued by a Canadian payer (such as an investment dealer, corporation, or trust) to a non-resident recipient who received dividends, interest, royalties, pension income, or other amounts subject to Part XIII withholding tax under the Income Tax Act. The withholding rate is generally 25% on amounts paid to non-residents, reduced by tax treaty; for example, the Canada-U.S. treaty reduces the rate to 15% on portfolio dividends and 25% on interest (subject to exemptions). The NR4 must be filed with CRA and a copy provided to the non-resident by March 31 of the year following the calendar year of payment. For registrants, proper classification of clients as resident or non-resident (collected as part of KYC and FATCA/CRS self-certification) directly determines whether an NR4 is required. A dealer that incorrectly treats a non-resident as a resident and fails to withhold and remit Part XIII tax is liable to CRA for the unremitted amount.
Source
Income Tax Act Part XIII; ITA s.215 (withholding and remittance); CRA NR4 guide