You CAN do an open landing or loft space in The Sims 4 and here is a quick and easy how-to:
Since the game recognizes room units and deletes floor tiles as a room unit, you need to make sure that the second floor area you want "open" is its own room. The space that you want to use on the second floor should be another room. You can subdivide it later.
(I have placed brightly colored floor tiles and paint so you can see the areas clearly)
Delete the floor tile of the area you want to be open to the first floor. Add your stairs.
Delete the walls around your landing or loft. You will see the floor tiles in the "open area" auto fill. Don't worry about it!
Demolish the floor tiles in the area you want to keep open. The ones the game auto-filled for you...those tiles. Delete 'em! They wont come back this time.
Add railing around your landing and divide up the space as you see fit. Decorate accordingly. You now have a loft or a two story landing!
NOTE: if you have a roofing section cutting through the open space of your loft or landing, it will show. The game uses room units to determine roofing collision. This means this technique works best in areas where roofing does not cut into the space.