Taking that 5D IV to Iceland is a big move but you will be happy with the results if you prep right. Honestly, weight management is the biggest hurdle for hiking. I am very satisfied with the Peak Design Slide Camera Strap Black SL-BK-3 because it distributes the heavy weight across your shoulder much better than the stock strap. Regarding the cold, those extra Canon LP-E6N Lithium-Ion Battery packs are essential, but you have to keep them in an inner pocket close to your skin to maintain their temperature. Here are a few other things I rely on for safety:

• SanDisk Extreme Pro 128GB CompactFlash Card 160MB/s for maximum reliability in freezing temps.
• Think Tank Photo Hydrophobia V3.0 35-70 Rain Cover since Icelands weather changes in seconds.
• Giottos Rocket Air Blaster Large to clear moisture off the lens. It works well for me and keeps the gear safe from the elements.

Just caught this. I did a winter trek once and learned that standard plates are a pain when your hands are frozen...

• 3Legged Thing QR11-LC Universal L-Bracket is cheap and versatile.
• Really Right Stuff B5D4-L L-Plate for Canon 5D Mark IV fits the body like a glove. The RRS is better for safety but the 3LT saves you cash for beer. Just dont forget to tighten it before hiking.

• tbh, I found weight distribution matters more than the actual weight of the beast.
• check shotkit for some real-world iceland packing lists. dm me if you've got questions!

I am always terrified of something happening to my gear on big trips, especially with the wind and volcanic sand in Iceland. One tiny scratch on the LCD can really ruin the resale value or just be super annoying to look at. I have learned to be really careful with the 5D IV since it is such a tank but still has some vulnerable spots.

• Expert Shield Glass Screen Protector for Canon 5D Mark IV vs those cheap plastic film ones. I would definitely suggest the glass. It actually feels like the original screen and wont peel off when it gets wet from the rain. The plastic ones always bubble up and look terrible after a few days of hiking.
• Nitecore UCN1 Dual Slot USB Charger vs the standard Canon wall charger. The Nitecore is way more reliable for travel because you can charge your batteries from a power bank in your backpack while you are actually hiking. The Canon charger is fine for hotels but basically useless when you are out in the wilderness for 10 hours and run out of juice. I would also be careful about keeping your camera in a sealed bag when you arent using it. The moisture there is no joke... I have seen peoples dials stop working because of the salt air and mist. Better safe than sorry when you are that far from a repair shop.