I don't think it's overkill. I never grew up with grandparents, they all died before I was even 5, so I never got to meet them. If they all get along, it sounds like a swell time, I know I would have loved that. If they don't get along and you think it will be awkward or there will be some fighting, I have an idea for you: why not have one set of grandparents walk one block, and the other set walk the other block? My daughter likes going with her cousin for a bit, so I let them walk a few blocks east together, and then when they get back, I walk the west side of the street with my daughter, that way we both get to share her, she gets to have fun with both of us, and my daughter gets to walk around, showing off her costume, and still collecting candy, uninterrupted. That might be a suggestion, especially if the grandparents are elderly and can't walk as much as your kid would want them to, just have them take turns!
I have seen families doing Halloween parties on their lawn, with scary music, ghoulish snacks, and handing out candy to trick or treaters, so that could be another alternative, have one set of grandparents linger and chat with you all after the party, and when the other set returns from trick or treating, have them switch places and have the exhausted grandparents sit and relax while watching the costumes, chatting, and handing out candy, and the other goes trick or treating with your son. When he is too tired, they can all sit and continue the party, bring out a cake, etc.