4 year old in her own room

How do I get my 4 year old to sleep in her own room? I haven’t set it up yet bc she sleeps w us in her own bed, I also have a 9m old that I cosleep with. I feel like she’s old enough to sleep on her own but I feel very sad/guilty leaving her alone in the other room, when our 9m old would still sleep with us. I feel like she would feel left out, sad, etc. or even scared as well! How do I approach this or go about it? My smallest is breastfed and still wakes up several times so she will stay with me. when she gets a little older she will also be transferred into the other room!