Thank you for writing. You certainly have your hands full with four kids. (I do too and I remember it well.) It must be very, very frustrating to be putting in so much effort and getting so little result.
The first thing I’d like you to do is find a moment to sit quietly and just take a few deep, long breaths. Then let’s take a look at the whole situation. You’ve told me your son has some developmental delays. That probably means that he will be delayed in toileting as well. You also said that he had a hernia operation recently. (4 months ago is like yesterday to a little kid.) He may associate pain with his abdomen and be fearful of doing a bowel movement. If so, he delays it as long as he can. He’s also made a connection now between a bath and peeing. Since he is delayed, you’re right to wonder if he knows that he isn’t supposed to use the tub now. And, on top of all that, toileting has become very, very important. In a household of a lot of kids, he may have found that a way to get your undivided attention and concern is around potty issues. Whew! No wonder this has become such a problem.
You didn’t mention your husband’s involvement. If he can, I think it might be helpful for him to take over in this department for awhile. He and your older son can show your little boy how boys pee in the potty. They can even make a game out of it. Suggest to Dad that he throw a few of those oat cereal circles in the potty and make it a contest to see who can hit them with pee. (Yes. Really.) Your older son can challenge him to a peeing race to see who can pee the most, finish first, etc. (in the potty, of course). The point for everyone is to relax about this and to make going in the potty more rewarding than not. The other point is to give you a break!
Your son will likely take another year or two to master all the skills of toileting. It’s okay. The fact that he is making some improvement even now shows me that he can. He’s just going to do it later. Let the teachers know that you are taking another approach and see if they can do the same at school. If you can take the stress off the issue, it may help everyone.
I wish you well.