Are you married? Do you have any other children? Does this affect them too?
I do not have any autistic children. I had one that has A.D.D. I have many friends with autistic children and the one thing that I notice the most is that it is important for them to learn boundaries and consistant rules. If they don't, they just do not adjust well in life.
I think I would have many nights of him crying in bed (his own bed) and love him to death when he wakes up and praise him soooooooooooo much!! I think I would just tell him that it is time for his big boy bed. Don't take naps with him or make exceptions. If he is sick...go to his bed to comfort him.
I'm sure you don't want to see yourself trying to handle this situation when he is 8 years old. At 4, he has already developed a safe habit and it will take a lot to undo it. It will even be painful to go through (I mean a heartache). You love him so much and want him to feel safe and happy, but you also need to prepare him for the next stages in life so he can someday function somewhat normally as an adult, right?
Oh, I wish you all the luck in the world. I have 6 children and my last one is 2 years old and still is sleeping half the time in my bed. I think it is more for me. :) I love it! But, we know that she needs to go to her crib and I am going to listen to my own advice and keep doing my best to help her grow up and still feel loved. :)
Take care and have a very Merry Christmas!