I have to agree with teaching your son to sleep on his own. I coslept with our son until he was nearly that age, and honestly, by the time he was four and in his own room, I was *done*.
I decided to tackle teaching him to sleep on his own with some support of some mom friends who I could talk with. They suggested the technique of putting him to bed and then, every time he popped up, putting him back to bed with no talking.
We chose to dedicate a week to this and for the first night, I basically camped out in the hallway outside his door. Yes, it was not fun, but I brought some books, crosswords, my laptop, so it was quiet and I wouldn't be bored or feel like I was missing out on anything. Kiddo popped out and I calmly walked him back to bed, just said "Bedtime now" once and that was it. He continued to talk and chatter away, and I just kept my expression neutral. He did about 4 pop-ups the first night, and got up once at about 1 to find us. (Yes, I did sleep in the hallway that evening.) "Back to bed". Took him back in and left him. He stayed in bed.
The second night was easier. 2 pop-ups and then he was asleep. I slept in my own bed that night.
The third night, we had one pop-up-- my husband took over duty that evening-- and that was it.
He is older now and still sometimes seeks attention in the evenings after bedtime. Usually it's because he can't sleep and might ask to look at books for a bit. If it's for attention, we warn him once that he will lose his 'stay-up time' the next evening (which is a privilege... he gets to stay up anywhere from 30-60 minutes after storytime for quiet play-- he's six) and that's it.
Plan to invest at least two or three evenings in teaching him to stay in bed and sleep alone. Once you have started, don't go back to laying down with him to help him sleep, even as a treat. Be steady. I say this with conviction: one of my former preschool moms really didn't want to upset her little pumpkin and lay down the law about bedtimes, so this was still going on when the child was well into 5 years old. YOU are the parent, one in a long line of mothers going back into generations beyond imagining. Stand firm. We have all had to teach our children that we know what's best for him and a simple "four year olds sleep on their own" is all the explanation he needs. He needs you to make this change because he isn't confident he can get to sleep on his own, so you will have to be certain about this for him. If we keep our cool, don't overtalk or threaten, just do this in a firm and loving way of being consistent in taking them back to bed, we do both our child and ourselves a huge favor. :)