every fourth year =every four years
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：
He comes here every three days (every third day).他每三天(每隔两天)来这儿一次.
You'd better plant a tree every five meters(every fifth meter).你最好每(隔)5米种一棵树.(树的空间忽略不计)
Write on every other line.隔行写.
every other day 每隔一天
every few days 每隔几天(注意“few”前没有“a”)
source:somewhere on the net