The phrase "fourth floor" refers to a level or story in a building that is counted from the ground level. In other words, it means the upper floor above the third floor and below the fifth floor if there are more floors in the building. The numbering of floors usually starts at the ground level which is considered as floor 1, followed by the second floor on top of it, then the third floor and so on until the highest floor. So, when someone says "I live on the fourth floor," they mean that their apartment or office space is located on the upper most completed level above the ground in the building.