Why a fourth rate offered awkward living conditions

Were service conditions aboard a fourth rate closer to a third rate or a fifth rate?

They could be worse in general because the ship had fifth-rate space but a crew closer to a third rate's size.

The comparison focuses on habitability rather than fighting power. A fourth rate did not provide the room available aboard a third rate. At the same time, it carried far more people than a fifth rate. That combination of constrained space and a large crew made life aboard less comfortable than either comparison suggests from armament alone.