Definition of fourth-rater:
(adj) :
(idiomatic, uncommon) Terrible, awful; less than third-rate.
(adj) :
(UK, military, nautical, historical) Of a Royal Navy ship of the line in the Napoleonic Era: having 50–60 guns across two gun decks, a complement of 320–420, and weighing approximately 1,000 tons burthen.
(n) :
(UK, military, nautical, historical) A fourth-rate ship of the line.