Does your 4-year-old still take naps? My granddaughter (3) was not going to sleep easily at night, and my DD decided to try shortening her afternoon nap, and then eliminating it completely for a while. I thought that would not be a good idea for a 3-year-old. But it solved the problem. She started going down pretty early in the evening. Believe it or not, she is now back to taking naps at daycare, for about an hour each day, and still is going to sleep okay at night, right after her bath at 7 p.m. I think sometimes they just need some help to establish better habits, and then can continue them without help.
Another thought is the obvious--his allergy meds. Those can either knock you out or rev you up. Is it possible they are acting as caffeine might, and reving up his system so he can't settle down? How about discussing this with your ped? Maybe there's something else he can give your little guy that wouldn't interfere with his sleep this way?
As for the poop thing--welcome to the club! Our 3-year-old granddaughter still has accidents in her big girl panties. I think they get very distracted by play, and don't remember that they need to get to the potty when they feel something about to happen. My DD has taped a potty chart right next to the toilet, and my GD proudly puts a sticker on it each time she does her poop there. When the chart is full, she gets a prize. It helps. But it also helps to keep reminding her throughout the day when we see the telltale signs of discomfort, which I'm sure every parent is familiar with--at least when you're with him. I know that's not possible when he's in school. But again, it might help him become self-aware during the week, with just weekend help to kind of train him.
No great words of wisdom there, but that's been our experience.