From The Collaborative International Dictionary of English v.0.48 [gcide]:
fourth-year \fourth-year\ adj.
of or pertaining to the fourth and final year in a U. S. high
school or college.
Syn: senior(prenominal).
[WordNet 1.5]
From WordNet (r) 3.0 (2006) [wn]:
fourth-year
adj 1: used of the fourth and final year in United States high
school or college; "the senior prom" [syn: {senior(a)},
fourth-year]