For my three kids, nighttime dryness was not a matter of just wishing it to be so - they weren't physically ready until ages 5, 8, and 10! For my girls, their systems just matured at the ages of 5 and 8. I did realize that the 8 year old needed to be in charge of her own bed, and she was ready to do so. By this time she was dry many nights and was able to make the leap to being dry all the time.
For my son, he was such a deep sleeper, and was wet every night until he was 10. At that time he was really motivated to be dry, so I got him a pee alarm. The results were amazing! Within 4 days he was dry every night and never needed a pull-up again.
Base on my kids, I'd say 4 is kind of young. He might not be physically ready to be dry at night. They all get there eventually. But you know your son best. If you think he'd be successful with being dry if you took off the pull-up, by all means try it. But if he still wets the bed, realize that he's just not ready yet, and be prepared to be patient for awhile!
If you want to try the pee alarm, you can find them at bedwettingstore.com. We used a Malem (which I actually got on eBay).
Good luck!