A 4 year old peeing on himself could be because the 4 year old is too busy playing and not wanting to get up to use the toilet.
Or the 4 year old could be shy about using the toilet especially when in someone else's house.
Or your 4 year old could be regressing after being potty trained.
I would put the 4 year old boy back in diapers full time until he stops peeing himself and tell him that when he stops peeing on himself then he can go back to underwear.
Changing diapers is much easier than changing clothes and cleaning up pee from furniture, bedding, floors etc.
It's normal for a 4 year old boy to still wear diapers whether full time or at night.
Some kids potty train as early as age 18 months and some kids potty train by age 2 or 3 years old.
Then some kids may not be ready to potty train until after their 4th birthday and most boys do potty train eventually before they turn 5 years old.
One of my boys potty trained just after he turned 5 years old.
He was learning to potty train by age 4 but he regressed and would pee in his pants so I just put him back in diapers and eventually by age 5 he was ready to use the toilet and stop wearing diapers.
All kids will eventually potty train on their own even if you do not train them as they will learn and want to imitate their parents and older siblings.
So just keep teaching them and reminding them about the toilet but don't force the issue unless they are 5 or 6 and not potty training.