One who is in fourth position or rank; something that comes fourth.
From 'fourth' (Old English 'feortha', ordinal of four) plus the agent suffix '-er' meaning 'one who' or 'that which'.
This word is archaic and rarely used today, but it reveals how English once freely created agent nouns from ordinals—you could theoretically make a 'thirther' or 'seconders' to name finishers in races.
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