Translation:The height from floor to ceiling is four meters.
You can differentiate by referring to the ceiling as 'himlingen' instead, and to the roof as 'yttertaket' or 'hustaket', but we usually call them both 'tak' without there being much confusion.
In the context of this sentence, I'd assume it to refer to the ceiling, as 'gulv' is also an "inside" feature of a building. It's the measurement that makes the most sense. If someone really wanted me to measure the length from the inside floor level to the outer roof (top), then I'd expect them to specify that more clearly.