I don't think it should be Kay's funeral. She's divorced Michael. She doesn't really mean that much to the family. By not being with Mike, the family is less concerned with her. It should be someone else...Maybe Vincenzo's mother or someone. Just not Kay. She is secluded from the family.

Or, it good begin with Vincenzo's wedding. It's the same in some aspects and different in others since it is the Don himself who is getting married. Of course, the meetings will also be part of it since they take part in the first three films.

He could be something like 7-10 years older and marrying. It can show that he has greatly matured since we last saw him. Albeit, he did seem to have some kind of maturity transaction between the beginning and after selected as the new Don of the Corleone family. He seems different, somewhat matured, when we see him at the Opera.

Or, it could be that Vincent is already married and some kind of religious event is going on that has to do with a son of Vincent.

I think we have some options to choose from.