In my sea kayak, I have come to love Greenland paddles. I don't even take the euro blades with me anymore. I have never, ever, used one in a canoe and think they are just wrong for canoes for a number of reasons, but it is all personal preference and what works for you.
I thought my first GP was too skinny and made my second one 5" wide. It came to be known as "the plank," and it was a chore to use. Over time, I came to most like a blade about 3.25" wide. The fatter blade was just too hard to pull through the water. Other kayakers using GPs would pull away from me, and I just couldn't pick up my stroke rate with that fat blade. You will need, relative to kayak use, a longer paddle, because your boat is wider than a kayak and you sit up higher than butt boaters. Length means leverage, and a wide blade will be even harder to spin on the end of a longer paddle. So, I think 4" is too much blade.
The loom on my paddles is around 18-20". That's the oval part in the middle, where I hold the paddle. I have two fingers overlapping the shoulders of the blade, where the loom transitions to the blades. As I paddle, my hands are often in the water as I bury the blades. I don't think that stroke can be replicated in a canoe. Should you make the loom longer, considering the width of the boat and the fact that you can't reach your hands to the water? It doesn't seem like having blades that extend during the stroke above the water and perhaps above the gunwales (i.e, paddling air) is helping anything. Too me, GPs and canoes make no sense, and I suggest you try regular euro blades if you must use a double blade. Maybe borrow or try a used set to see if you like it.
When I use a euro double blade in the canoe, a lot of water drips into the boat. Some paddlers seem to stay dryer than me, but if you are using torso rotation the off-side of the paddle ends up over your boat, and you get the dripage. GPs drip way more than euros, so if you use one in the canoe, get used to a wet lap and and a substantial bilge puddle.
GP brag, unrelated really: I am the
only entry to ever use a Greenland paddle in the 16 years that Baltimore's Kinetic Sculpture Race has been run. But I used it like a single blade.