Can you work out from their comments which floor they each live on?
A: 'I live three floors below D.'
B: 'I live on the seventh or eighth floor.'
C: 'I live on an odd-numbered floor.'
D: 'The floors we each live on are an equal distance apart.'
According to D, the floors they each live on are an equal distance apart.
If they lived two floors apart, A's comment doesn't work, and if they lived three floors apart, a nine-storey building would not be tall enough.
This means they must live just one floor apart from each other.
Continuing on from hint one...
A's comment tells us that A lives on the lowest floor of the group and D lives on the highest floor.
Continuing on from hint two...
B and C live one floor apart.
B says he lives on floor seven or eight, but if he lived on floor seven then C would have to live on an even-numbered floor, so that can't be correct.
This means B must live on the eighth floor.
Let's go over the facts.
The four people each live one floor apart, with A at the bottom and D at the top.
B lives on the eighth floor, so D must live higher than this on the ninth floor.
Now you just need to go over the comments again to figure out where the other two live.
The answer is as follows.
A=6
B=8
C=7
D=9