What a difficult position for you: your son fights getting changed, but can't/won't keep himself dry. It sounds like there must be a tremendous amount of tension around the subject for both him and you.
I wish you had said how long you've been trying to get him trained now (if your daily "prodding" has been going on for months, the fix may take longer). But no matter when parents start the process, a child will seldom be truly trained until he is ready. And when they are ready, the training happens quickly, sometimes in a day or two. Earlier, it's often really the parent who get trained – to get the child to the potty in time. For boys, who can be considerably slower to reach physical and neurological readiness, training is sometimes completely pointless before the age of 3.5 or sometimes 4 or so. Pushing a child will often make him resistant, and everything will take longer.
You don't say how long you've been working at this, but if it's been months now, your son might be resisting the pressure he feels. This could spill over into anything relating to his pottying, which might include getting cleaned up after.
What if you were to try a tactic suggested in the wise little book How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. I've been using these techniques with my grandson since he was around 2.5, because they are so effective and reduce tension so well.
The authors would suggest that you trouble-shoot the issue with your very bright son some time when he's NOT needing to get cleaned up / changed. Tell him that you know he's not happy about having to get changed, and you're not happy having to change him. LEAVE SOME SILENCE, and listen to his response. If he agrees, suggest that he might have some ideas about how to deal with the problem. Again, leave some silence. Watch and listen carefully to whatever he does say, nod and say "mm-hmm" and "I see," giving him lots of room in which to get his own version of the current situation out.
Whether or not he says anything useful (and he very well might, since he's bright), just commiserate with him sincerely about what a drag it is to need to get cleaned up when he's wet or dirty. Make sure he knows you really care about his experience. Then, ask him for ideas about how to solve this ongoing problem. Get a sheet of paper, ask him to bring you a pencil or crayon, and write down all his thoughts. Most kids will be stunned and impressed that their parents will actually take the time to listen, and then WRITE the child's ideas.
Ask him for any and all ideas he has about how to change the situation. You are brainstorming here, so don't judge or discount ANYTHING at this point. Add your own ideas to the list, including a couple of goofy ones if your son is throwing out some silly ideas. This process can actually be fun, and a touch of goofiness might help him relax and enter the brainstorming more creatively. Be sure to leave time/space for all ideas to make the list.
Then go through and consider which ideas will be most workable. Be sure at least one or two of your son's solutions stay on the list, even if they are not your preferred choices (remember, not all 4yo's are able to be trained for a few months yet). Perhaps he can change his own pullups when he's only wet, and allow you to clean him up when he poops, for example. You might be surprised at the ideas that end up on your list. And if your son has come up with some potential ideas to try, he will be much more invested in making them work.
This is only a thumbnail sketch of how/why this approach can be so effective. I strongly suggest you purchase this inexpensive, easy-to-read, and brilliant book for yourself. You'll reach for it often. Meanwhile, keep the messages around potty training positive and practical and light. For example, when he's peeing, Daddy might observe that he's glad he can use this simple, quick method of getting empty. Or he might use floating cleerios for target practice. If your son enjoys potty books or videos, or puppet shows, those are great devices for keeping the messages positive.