First Aid Kit appropriate for conditions.

Rather than food that must be heated and soaked, I would pack things like peperoni and cheese that can be either heated or eaten as is - take plenty of calories for cold weather. I think I would make that a large fuel bottle. Melting snow takes a lot of gas.

You will need to stay hydrated in cold weather. At least a canteen, but even better would be a thermos of at least a quart capacity with a wide mouth.

Worst case scenario - you need some sort of shelter and probably a sleeping bag, although I understand snowmobile suits are really warm. Just be sure you can spend the night out if you have to.

What about tools and repair kit? I would definitely carry a multi tool rather than merely a pocket knife. Maybe it's just me, but I would include at least 60 feet of nylon rope (you always need a bit of rope) - versatile item that can do a lot.

How large is your large back pack? - capacity in cubic inches.

For a four day trip, I would plan on wearing one set of thermals, with another pair reserved for emergencies. This would free up space. Toiletries would definitely be minimal - skin oils provide some protection from the cold.

I have never snow mobiled, but I am surprised you don't have more capacity. You are not carrying any more than a touring skier, who would also carry a large back pack.

Sounds like an interesting trip. Have fun!