Just like how zone 4 in the United States is different when comparing New Hampshire and Minnesota, so too I assume your comment about "Canadian Terms"

Obviously the prairie provinces will have more winter wind issues that the forested areas around Toronto. How much snowcover do you get in your area? Is it all in the spring so that it is heavy and wet or is it consistent across the season? Do you have moderating winters because you are on a Great Lake?

Having said that, if you are questioning the hardiness of 'Green Giant', I'd be wary of using it. It is a hybrid that had Thuja plicata as a parent, which isn't overly cold hardy.

I would suggest a cultivar of the more cold tough species Thuja occidentalis (zone 2-7) , like 'Techny', 'Mission' or 'Nigra'. Or, any other hybrid cultivar that does have some T. occidentalis in the parentage.