you aren't "using" SDs room. you are providing a room/space for all your children. which includes SD. It is not always feasible for the "oldest" to get their own room. my SDs have shared a room through 3 moves. This last move was when we bought our house. When i was pregnant (before we knew the gender) We had told SD, if the new baby is a girl, when it's time to move them out of our bedroom, you will get your own and your sister and new baby will share. If it's a boy, you'll have to keep sharing and he will get his own room.
well, he is a boy and SDs still share. little different since they come and go together. so their room is not in use when they aren't here. 50/50. SDs are 4 years apart. about 10 years in maturity. Literally yesterday, i found a loft bed on sale with a desk underneath. We are picking it up later today. (SDs are at their moms) so I can make SD13 a more private space in their shared bedroom. younger SD is a terror and has many behavior issues. We are hoping to save up $ in the next 2 years to add on to our house and make SD a room.
but, I do agree that a TALK with SD about the change is needed, not an ASK for permission. I don't agree that the bedroom should be used for 4yr old "just for sleeping" they also need their own dedicated space even if its just a nightstand with a couple toys and books. Even though SDs have shared a room, they've always had their own bed/nightstand/side and then a "shared" area with like board games and things they both "own".
I'd say, if you have a decent relationship with SD let her pick some things or help design her "space". area rugs, especially for the 4yr old, even if its just a solid color one could let them know this is "their area" to play or keep their things. Higher up shelves could help SD keep her stuff in her reach but not in 4yr olds.
They also make sound machine clocks where you can set times/colors. you can get one for 4yr old and maybe work with them to learn a schedule. (even when SD isnt there) like "green" means you can get up. "yellow" means if you're awake, you need to play quietly or go to the living room/your room etc.