When both of my kids was 4 yrs old , i toilet trained. Make sure they pee in the toilet before bed. There were many times they peed on the bed during sleep, eventually they learnt about going to the toilet after that. So I have to constantly remind them that if there is urge, must wake up go toilet peepee.
So , yeah, expect many peeing on bed to happen before success happens.
Lena Tan