In mainland China the ground floor is counted as 一楼, so 四楼 = British 3rd floor / American 4th floor. Because 四 sounds like 死 (sǐ, death), many hospitals and some residential buildings skip the 四楼 label, jumping from 3 to 5.
grammar
Floor number pattern: [number] + 楼. 一楼, 二楼, 三楼... Above 10, use 二十楼 not 二十层楼 in speech. 层 is the formal counter-noun form: 第四层 = 'the fourth floor/story'.
