Hello, dear client, and thank you for this wonderfull contest. Your building and apartment are beautiful so i tried to adapt this great space to modern life of two with respect for original design premises.
Since you two live alone and like large spaces I opened all the 4th floor for one great living area. I moved kitchen from the entrance of living and dining area so now you have one spacious room to fit your new furniture and accesories. Following the original architecture elements I opened another arch portal which was closed so now you have two original floor-to-ceiling openings that are just opposite to high windows. The same reason is why I closed the third door between so it doesn't corrupt the layout.
Small room at the 4th floor is now large kitchen area (with some minor demolishing of non-bearing walls) accesible from the living area and from the entrance hall. Almost whole existing kitchen cabinetry can be put here and only the far right black part of it should be adjusted for this wall is 20 cm shorter. My suggestion is that you insert the lower cabinet storage in this black part since there is no radiator here and to add some modern marble kitchen island that will calm this existing contrasting kitchen.
Toilet beneath the stairs I turned into laundry and storage area and added new toilet accros with small corridor for these auxiliary spaces.
All the 5th floor is one great bedroom zone with three areas - wardobe (walk-in closet), bathroom and sleeping area.
All the best,
Vanja Kostic