I have a four-year-old son. He won't get up and use the bathroom, instead he'll tell me when he has to go and I'll take him. There are days were he'll pee on himself when I lay him down for a nap - he'll just lay there and pee all over himself without getting up and using the bathroom... And also at night, too, there are times where he'll stay dry throughout the night because I take him, now all of a sudden he's all wet even when I wake him up to use the bathroom.
Something we did with my son at about that age, when he had been potty-trained for a bit over a year but was still wearing and wetting a pull-up at night: I would of course limit his fluids for the last hour before bed and had him go to the bathroom right at bed time. Also at first I tried to check him frequently after he would go to sleep, to find out when the wetting was taking place. Once I established that he was usually wetting around 11:30 to midnight each night, I would go in and get him up around 10:30 each night, right before I went to bed, and take him to go potty. He was so asleep that he didn't even fully wake up. The thing is just that sometimes their bladders are still a bit small and they are still young enough that the urge to urinate simply doesn't wake them. I did this waking him up for a potty trip right before I went to bed for just a few months, and then he started doing it on his own.
This is a common problem, we fought this with our daughter over a number of years. I don't know that you can do much training, as they are asleep. If they're not really asleep, and are just too lazy to get up, well that's another thing, but for the purposes of this question let's just assume they are asleep when it happens.
A child has to be trained to unconsciously know their bladder is full, and refrain from wetting themselves. This takes practice. You can do a little training during waking hours by filling them up with water, and encouraging them to experience what a really full bladder is like and not just urinate at first urge.
As parents, In order for us to stay sane, we kept a ready supply of "Pull-Ups" and never made them feel bad for any accidents. They will eventually grow out of it.