every fourth year =every four years
every 與基數(shù)詞、序數(shù)詞、other 和few 連用,表示時間或空間的間隔,意為“每……；每隔……”.如：
He comes here every three days (every third day).他每三天(每隔兩天)來這兒一次.
You'd better plant a tree every five meters(every fifth meter).你最好每(隔)5米種一棵樹.(樹的空間忽略不計)
Write on every other line.隔行寫.
every other day 每隔一天
every few days 每隔幾天(注意“few”前沒有“a”)
source:somewhere on the net