I wouldn’t worry about it, she’s not going to remember it anyway! Just explain your not doing a party cos your going to the zoo instead and that’s already booked and paid for xx
Keep it small and do under water theme at home I completely agree that doing with classmates when there older is better idea as then you will be able to give them what they want and need especially as there will be able to tell you who there would like, We have our girls in our garden ever year so far and she loves it we have paddling pool out some football games out and just garden fun.
It's months away, so there's a possibility she will probably forget. I wouldn't worry about a party for her 4th birthday, we don't plan on having on, just a family gathering, there's plenty of time for the chaos of a party