C.,
I had been experiencing a similar thing with my 4 year old. He refused to sleep in his bed, and ultimately, we let him sleep with us, which had been going on for way too long.
I had read a book by Elizabeth Pantley, and in rereading a specific section, she mentioned something called the "Morning Fairy". You can revise it in whatever way to meet your needs. One night, he asked to go to sleep in our bed and I told him that I had spoken to someone named the Morning Fairy, indicating that she would leave a present (a small one, like a Match Box car) if he could sleep in his bed and stay there. Right away, he jumped out of our bed and went into his room and fell asleep in his bed and stayed there. Any night that he was a little scared to sleep in his room, we just gently reminded him about the MF, and that was all we needed to do.
After about a month, we gradually weaned him off of this, though as an incentive, the MF makes surprise visits now and then. I had resisted giving prizes for rewards, but the power of them was quite amazing.
Just an idea. I know it worked wonders with my son.
C.