I had five boys and one girl. The boys were potty trained as they were ready. One of them at 18 months, most of the others by two years old. My daughter, however, had a mind of her own. When she hadn't potty trained by the time she was three, we started concentrating on it like you are. It did not help. She didn't mind sitting on the toilet because we read to her. She knew that she could get me to read to her/ spend time with her if she was on the potty. A few months later, we went camping for a week. I brought a pair of underpants, but mostly diapers for her. She walked up to me in the middle of that week and said, "Mommy, I want my big girl panties. I'm not going to need diapers anymore." And she didn't. No accidents. One of my boys had a hard time training and had accidents and felt bad about it. I, personally, think he was in a stressful situation, which shows itself in potty training a lot. He said a prayer (we believe in God) and was immediately better. Now, in your case, especially with that 4-year-old at your home, that could cause a major setback. I think that if you are more casual about it, and tell him that whenever he is ready, he can wear big boy shorts instead of diapers, that he will tell you when he is ready. I think that he was ready before, but that child made it stressful for him. Rather than punish that child, though, I would make it a "no big deal" issue. That child should be told that he will decide to be potty trained when he is ready, and that no one should laugh at him until he makes that decision. Good luck!