S.R.
Hello L.. I wish you the best of luck with this challange. With my four-year-old, I put her to bed (she no longer sleeps in her own room, but on the floor next to the baby) and when she wakes up during the night, which she does several times a week, she usually comes into our room and sleeps on the couch. What I did with her when she was younger and climbing into our bed every night was tell her she had to sleep on the floor or go back to her own bed. Just not enough room for all of us.
It can be a challange, but if your pick your plan and stick with it, your little one should be going to sleep on his own in his own room in a matter of weeks. I suggest the Supper Nanny way where you just put them to bed, and when they get up, just walk them back to bed without even saying anything. Best of Luck!