Some American buildings don’t have a thirteenth floor. By the same token, some Korean buildings don’t have a fourth floor.
These buildings are more than four stories in height, of course, it’s just that the fourth floor isn’t designated with the numeral four. Rather, the fourth floor is designated by the letter “F” — standing for “four,” I assume. It’s even more common on the digital floor readers on elevators, which flash an “F” between floors three and four.