I'll apologize now if I am providing steps not required, but thought it easier to do start to finish.

1. Select T4's process forms

2.Select names and Review, print and select Efile for T4's  (say yes to proceed if you have unsuccessfully tried before) and then send.

3. This will take you to the CRA website. 

4. Select Internet File Transfer

5. Agree and Sign in to CRA 

6. Before you attach the file. Go to your Documents (or wherever the XML file is saved)

7. Right click on the file and select Open With and choose Notepad.

8. Change the TransmitterAccount number to match the number you just used to sign in. (mine was the 5th line down)

9. Select File (top left) and then Save (not save as).

10. Go back to you Quickbooks page and Select File (select the file you just amended and saved)

If all good you will get a green attachment and you can hit Next for your confirmation.

Done