Please see this answer from CarolynB.
To enter the NR4, treat it as if it is a SSA-1099 and enter it in the Retirement Plans and Social Security section, under ‘Social Security (SSA-1099, RRB-1099)’.
Then, go to the Deductions and Credits section and select Foreign Taxes under ‘Estimates and Other Taxes Paid’ to enter any foreign tax you already paid to Canada on that income
For more info specific to the US/Canadian tax treaty, please see http://www.irs.gov/pub/irs-pdf/p597.pdf
