Where would armored cruisers fit in the sailing-era rating system?

Were armored cruisers the steam-and-steel equivalent of sailing-era fourth rates?

Only approximately. They fall into the third or fourth rate depending on the cruiser's size and form.

The question compares armored cruisers with the sailing navy's fourth rates. A single direct equivalence is too simple. In the rough analogy given here, first- and second-class battleships occupy the first two levels, while armored cruisers fall into the third or fourth depending on their size and form. Administrative labels changed with technology, and ships within the cruiser category could differ substantially in fighting value and deployment role.