It is not easy.
Similar situation earlier for self, it took ages to resolve to where manageable.
You need others nearby who can support you when through things becoming difficult for both you and your nephew.
As only 4 best to re-assure your nephew is right thing to ask for help, even to ring others, when feels need for it.
Even at 4 years, teach him how to dial 000 for Fire/Police/Ambulance, be sure he knows this is OK when-ever feels he needs their help,
Also teach him to call Lifeline or KidsHelp and chat to them, make sure you get him to do this when he wants talk to someone about something so it becomes second nature.
Make sure he knows places where people safe for him to walk and talk with, so when feels need knows can do.