Overnight Ferry Stockholm to Helsinki and Tallin to Stockholm

Expect a pretty comfortable bunk bed but not much standing-around space. Top bunk can be folded up when not in use. See also https://www.tallinksilja.com/cabin-info or https://www.sales.vikingline.com/aboard/cabins/, with information by name of ship. Enjoy the archipelago view when departing Stockholm. Either regular or buffet dinners are available and have been discussed a bit in this forum before. If the sea gets rough, I think in bed is the best place to be.

To be honest, I've only taken shorter, daytime ferries in Scandinavia. But do you understand that the historical growth of this product was originally based on Partying and Duty-Free Shopping, in a past, pre-EU environment? I don't think they envisioned that four parsimonious backpackers would jam into a windowless cabin to save one expensive hotel night in Stockholm! (Not talking about you personally, just about the boat ride product ... ) I'd rather have the night in Stockholm, but I can afford it. And I get seasick easily. Presumably you are doing this in summer, when the seas might be a little less active than winter.

We have an inside cabin of 4, 3 adults will occupy the room

It is basic. Four beds ... two upper and two lower and a small bathroom. You won’t want to spend time in the cabin except to sleep. The ferry website should have photos of the various cabins.