Hello jbirston,
T4 season can be hectic, but I'm here to help make sure it runs as smoothly as possible for what you need to do in QuickBooks Desktop. It's important to me that you get all the details right, and based on what you've said, I have a few suggestions.
QuickBooks Desktop's Payroll add-on will only produce T4s for employees that have been entered in the system. When you have an employee you need a T4 for that hasn't been a part of pay runs, you'll need take care of that outside of QuickBooks by manually filling out a form from the CRA website.
I recommend checking in with an accountant to learn how this might impact your books. It's also worth checking in with an accountant or the CRA to find out how to best handle this situation as a non-employee may need some other kind of form than a T4 since those are for employees of a company. I found this article from the CRA that might help: T4 - Information for employers
To find an accountant that knows the ins and outs of QuickBooks Desktop, take a look at our Find an Accountant page.
I hope that helps!