Welcome to the QuickBooks Community and thank you for reaching out! QuickBooks is a versatile program that enables you to get your work done faster. I'll be happy to provide some information on adding your SIN to your T4.
There isn't a way to adjust or add Social Insurance Numbers that show on the T4 slip, directly from your QuickBooks Online. However, you can adjust it on the CRA website and print/download copies from there. If you need to adjust any amount on the T4, I recommend reaching out to the Payroll support team outside of the Community, They'll be able to verify your account details in a secure setting, as well as view your screen to better assist you. They will also be able to verify if this could be fixed in the back-end. You can reach them by following one of these methods:
Schedule a Callback or start a Chat: click (?)Help in the upper right > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat. Social Media: Facebook and Twitter
If you have any other questions, feel free to reach out here.
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