Help with 4 year old sleeping by himself

[acplayer]

My girlfriend, who's a single mom, has a 4 year old son sleeps with her on a consistent basis. We're about to move into an apartment in two weeks and her son has been used to sleeping in bed with her because he is afraid of his room which he shares with another 3 year old. I'm afraid that his new room will "scare" him and he'll wander to our bedroom, wake us up and try and sleep with us.

In the past when she tried to lay him down in his room, he'd get up crying at her bedroom door, she'd put him back in his bed, and eventually within minutes or an hour, the same thing would happen. Eventually, she let him sleep in her bed, which is still happening now.

I've read suggestions and articles but I need to know what everyone else has tried and that has worked. I'm very new to this, I appreciate it!