everyfourthyear=everyfouryears
every与基数词、序数词、other和few连用,表示时间或空间的间隔,意为“每……;每隔……”.如:
Hecomeshereeverythreedays(everythirdday).他每三天(每隔两天)来这儿一次.
You'dbetterplantatreeeveryfivemeters(everyfifthmeter).你最好每(隔)5米种一棵树.(树的空间忽略不计)
Writeoneveryotherline.隔行写.
everyotherday每隔一天
everyfewdays每隔几天(注意“few”前没有“a”)
source:somewhereonthenet