From WordNet (r) 2.0 (August 2003) [wn]:
4th
adj : coming next after the third and just before the fifth in
position or time or degree or magnitude; "the
quaternary period of geologic time extends from the end
of the tertiary period to the present" [syn: fourth,
quaternary]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
4th \4th\ adj.
1. coming next after the third in position
Syn: fourth, quaternary
[WordNet 1.5 +PJC]