Some cabins have a bunk bed that folds up into the ceiling. This makes it a cabin that sleeps 4. If there are pipes, electrical cables, etc. in the ceiling so that they couldn't build in a bunk bed in that cabin, it will be a cabin that sleeps only 3.
Your room steward will raise/lower the bunk if needed.
Come cat 9 on deck 2 sleep 3, others sleep 4.