I think someone else looked into this last year. There are quite strict rules about taking pets on ferries. As Souffle says, he would need to be left in the car and even then you'd have to check the ferry company regulations first.
As it is so last minute, it is difficult. So you have four hours travel, then 2 hours on the ferry. How much travel is there after getting off the ferry? Your hammy could sleep through most of it but after 6 hours might be awake and active.
Once he's in his cage at the other end, and you're there (familiarity) he should be ok. I really depends on how long the total journey is I guess, each way.
It would be better if you can, to ask someone you can trust to just have him for the 5 to 7 days you are away. All they need to do is put food and water out each day. Although I know it's hard finding someone at short notice and someone you can trust. Whereabouts are you in the country?
If there is no other alternative, then as Souffle says they travel well if they have their own cages. The logistics woud be tricky. You'd need to bag up all the cage contents and set it all up again at the other end (otherwise everything goes flying when you have to brake hard!). I took our hammy on a half hour journey and his cage in the back and forgot to take the food bowl and water bottle out - there was food and water everywhere afterwards and all the substrate got soaked
I had him on me knee in his pet carrier with a blanket over while we were driving so it was nice and dark in there.
You would need a pet carrier though - you could take the cage with you to set up at the other end, but if he travelled in the actual cage it could be a bit too risky going over bumps and braking.
So sorry about your Grandad.