Official answer on whether registration is mandatory for the summer/spring semester

“If you are willing to take a break in Spring/Summer term, you need to request a leave of absence, and then get the approval from your graduate coordinator and submit it to me. I am attaching the form here. Please let me know if you have any questions. ”

So theoretically you can take time off during the summer/spring semester, but if you don't finish the remaining 4 courses in the summer/spring semester, you'll need two semesters (Fall + Winter) to complete them, which would add 1.2w CAD in tuition for one extra semester.