Try Benefiber. Dulcolac is very violent. Also, get a decent small toilet ring, so he is not just hanging in midair. There are even some double toilet seats (we got ours at Home Depot) that screw on instead of the adult toilet seat, and the kid-size one goes up out of the way into the lid, but can be pulled down for the child to use. I would keep him in diapers or pullups until it stops hurting him to poop, and encourage him gently with the cool new toilet seat. This can't be handled with "tough love," as it's partly a genuine physical problem, not defiance or just some kind of emotional stunt.
Potty training is so hard. There is probably a combination of things with your little guy. Behavioural and parental, BUT also physical. The fact that he suffers from constipation is a big deal in the potty training world, and you absolutely can't deal with the rights and wrongs until you get the poor guy regular. No amount of good parenting and potty training practices are going to work if it hurts him to use the potty and he is afraid. Stool softeners are not pleasant, they can cause cramping, as can the constipation itself, so if he isn't feeling 100%, he's not going to act 100%. Once you get him regular, and feeling better, toss out the diapers and be sure that he has something to rest his feet on when he is using the potty. Leverage for little ones is very helpful. :) Diet is the most effective way to deal with constipation, but he's 4, so you'll have to get mighty sneaky with the fibre :) Good luck!
Its a good idea to remove the 'nappy' diaper...you are enabling the issue by using it, at 4years old he wants big boy pants . Try less of the persuading, allow him to be in charge a bit more, suggest a visit to the bathroom then leave him, it is possible out of your concern you have created an anxiety at using the potty. If he is constipated it hurts to go, so work on a good diet with fibre to help .Is he taking a good food supplement ,anti oxidants Vit C and B Complex? they also have probiotics for children to help regulate them .Take him into the store with you let him chose nice veg ,fruit,food he would like to eat .And for the moaning , let him , do not react to it, once it gets no attention it will stop, focus on his positive side and praise him when you see him doing something right ..No one ever said good parenting was easy but its worth while ..