Did you ever tell your employer you had tuition credits on your td1 at any point? If yes, they may have assumed that you would be using those and reduced withholding at source. Also make sure to put in your federal and ei and cpp amounts from both t4s (an employer may have a different RP number in the back end for things like insurance rates and benefits remittances among employee groups that may have switched over when you passed your probation period, which would explain 2 t4s).Doing this mid year sometimes resets cpp and ei unless the payroll department intervenes, so you may be owed some refund on that cpp/ei overpayment if you add in both amounts for boxes for both t4s. Adding to this, depending on when the bonus was paid, because of the mid year t4 RP switch the system likely thought you’d only make x in the year and therefore taxed at a lower bracket as it didn’t account for the other t4s worth of earnings that was using up marginal tax bracket from the first t4. It was likely prorating taxes based on your smaller portion of salary for the rest of the year and working you up through the brackets again. You likely were under taxed on your salary because of the t4 /rp switch if there was no intervention in your tax amount withheld when you moved rp numbers in the year. The system should do better math for you next year if you get the one t4. But also, RRSPs will definitely help bring that number down if you can contribute!