First of all, try to distance yourself a little emotionally. You are right to some degree that "it's a control thing", but that doesn't mean he is trying to control you. If he is using this for control, it's to feel a sense of control in his own life, which all humans want to have. Since I will trust that as his parent, you know him better than anyone, if you think it's control, start giving him more say in other ways so that letting go of this one isn't giving up more control in his life. Allow him to gain control in other areas. This doesn't mean let him run your life. It's simple things you can do. Ask if he wants to put on coat or shoes first. Does he want to put on the left or right one first. Is he wanting to do it himself or would he like your help. You can add in choices 100 times a day in tiny ways that make a small kids feel empowered.
Another way to ease control issues is what I call "fair warning". I give my kids a play by play of what comes next all day long in 2-4 steps ahead. It can start at wake up. I might tell them good morning and then tell them that after they go potty we can have breakfast and get dressed. This seems small and maybe even obvious to a 4 year old, but it is "fair warning" of what the next 3 steps are. When we are sitting down to breakfast I will reiterate that after this we should get dressed and then mom needs to do a little laundry and I'd love them to help me so we can get done and have a little time outside. This again stays several steps ahead. It merely acts as a reminder to them of what is coming and helps them better predict what will happen. It's easy to lose track at 4. When doing laundry you can tell him you only have about 30 mins outside before baby has to nap and he can wash up and play with blocks or whatever he loves while baby naps. Then when outside you can give him more cues as to how much time is left, etc. A child who knows what is going on and when feels like life feels more in control even if they are not the one calling the shots.
If he is cognitively typical of 4, indicates that he knows when he is pooping (such as hiding, etc), then you can have a simple conversation about poop. You can ask him where you poop, where his friend poops, where grown ups poop, etc. You mostly just ask him the right questions so he is telling you that people poop in toilets. Also ask him who he should ask if he wanted to poop on a toilet. Ask him for names of people that might help him, you, maybe even grandma or others. When tell him you are so glad he understands all that. You are glad he knows where the toilet is and uses it for pee, and he knows who to ask if he needs help if he decides to poop on the toilet. Tell him it is his body, and he knows when he needs to go, and he understands that if he wants to use a toilet, how to do so. And then tell him you have decided that it's entirely up to him whether he chooses to use it or not. You trust him to make a good decision on this.
Then, when it's time to change his bottom, you have the second part of the talk. You remind him of that earlier talk and tell him that you mean all of that. It is totally his choice. Be very very very sweet. You must be kind. You have to understand he is going to likely hate what you say next. It is okay for him to hate it as long as you are acting in love. You cannot be angry, upset, frustrated, etc. It will not work if you are having negative feelings or seeing this in any light as it being a penalty to him. So be in the right place in your heart and emotions when you do this. You then tell him, that since he is so capable of so much, it is time for him to learn how to change his own pants. This will allow him total freedom to choose whether he wants to continue to poop in a pullup or use a toilet. You tell him this. It is your way of giving him ALL control. And then you have him do it. It's going to be gross and messy and he won't be good at it. This is also why you have to be in the right mind frame. You need to lovingly and patiently assist him verbally in the steps, stay calm no matter how he reacts, and be supportive in your words. You can tell him it's okay that it's not going very well, no problem, you can help him take a bath after if that is needed (and it likely will be needed) but that learning is process and he is going to be great at this with a few practices.
I really want to impress that it has to be done sweetly. If you are consistent, sweet, kind and supportive, I haven't seen a child so far this hasn't worked for relatively quickly and I use this before this age even. I find about 3 or so is usually old enough for most kids (I do think every child is different though). I usually have seen the same pattern emerge through this process. The first reaction is total sadness and telling me they cannot do it. The second one is begging and bargaining. The last one is anger and then usually it's over. While they are upset I just keep telling them it's okay, change is hard. I commiserate that I too hate the mess and I understand why they hate it like they do. Do NOT ask them to use a toilet during this time, at all, ever. This is all about allowing them to come to terms with making that choice.
And I don't know that I would feel up to this with a 5 week old. I suppose that is different for each person. My experience (in my recollection) with one that small is absolutely no real sleep routine yet. I am not saying wait ages, but it might be easier for you to do when the tiny one is at least consistently seeing night as night and you are not so exhausted. If you have someone to assist you during the process (like a mom or sister, etc) that would help, as the time it takes a 4 year old to clean themselves can be a tad lengthy. It is not a long process (meaning before they choose to use a toilet) but there can be a fair bit of time involved while they are working through it. I don't find this a particularly stressful thing to do, but I also have never done this while caring for a newborn.
