I'm very sorry your stepson lost his mother so suddenly and this is so sad. IMO I feel he should go to the funeral and be around his mothers family. Even though he's 4, its still important for him to be exposed to this grief process. Sheltering him from this will be more confusing when he tries to understand why he'll never see his mother or why she won't come back to pick him up. Here's an article on children and grief from PC.

Children and Grief | Psych Central