As long as the cabin description doesn't say 'shared,' you'll have the whole cabin to yourself, even if you're just one person booking a 4-berth cabin.
On a few routes, shared cabins are available. If you book a cabin that says 'shared' in the description, you're booking one bed inside the cabin, which you will share with other passengers.
If you book multiple berths in a shared cabin, they are not guaranteed to be in the same cabin. So, if you want to make sure that you'll be in the same cabin as the rest of the passengers on your booking, we recommend booking a whole cabin.