give the child the facts and run down of what will happen at the funeral,
give him the option of 'helping' - welcoming people, handing out order of service, being chief bubble blower, guardian of the flowers etc then let him decide

if he wishes to go try and appoint an unattached person who he trusts to act as a distraction when you/his family are too distressed to comfort/reassure or distract him

the jobs mentioned above will help him feel a part of the service rather than it feel strange or scary to him

when my partner died the children did not want to see the dirt get thrown onto his coffin so we all threw flowers from his garden in instead.

remember there is no set way of holding a funeral, it is after all a time for the children to say good bye too so let them do so in their own way, do not expect them to conform totally to adult expectations of a mourneful funeral. if they want flowers instead of muck that is ok, so too is rocus play at the wake, they are still kids after all.

remember to remind him that some people will be very sad and might cry, but that is ok, if he feels like crying he can that is ok but if he doesn't feel like crying thenit is ok he doesn't have to, it is his choice either way. because some children see others crying and think they have to even if they don't want to.

put a few noisless sweets in his pocket so he can chew them to calm down if he gets upset or if he decides he is hungry half way throught the prayers! like wise try not to give him too many drinks in the hour before the funeral and that he empties before you set off! there is nothing quite like a loud proclamation of "i need a wee" breaking the silence of the reflection time! a little note book and pen are great distractions you could have on hand too, and it is perfectly ok to take along a favourite teddy too, he is four after all and favourite teddy will be a big comfort for him and talking point when he meets all those strangers, much better to be asked teds name or recieve a compliment about your favourite ted than be greeted with oh poor thing or it's so sad.....