and check out that no other older child around him isnt spitting ...children do copy and older children love to teach them to do stuff like this ...check out friends and family...anyone he plays alone with ...
He needs immediate punishment, not something so far away as a vacation in a month.
The minute he does it, he needs to be removed and a parent needs to come pick him up and it won't be fun.
Four year olds really don't get delayed punishment at all - it's too abstract.
I think it would be cruel to keep him from family time for something as minor - and fixable - as spitting.
Disciplining a child does include taking away vacations. A child will think he can get away with anything if the big fun things aren't taken away during punishment. I understand you don't want to be punished too, but that's part of the process. It would be best if you support their decision rather than undermine it. Sorry, but children need to understand the consequences of their actions.
p.s. - you say he's not a mean child and doesn't hit - but the spitting may be just a warm-up, so to speak, for hitting. To a 4 year old, spitting does not justify getting hit in return. All it will do is encourage him to use physical behavior to try to change other people's behavior. I think at this point, you should worry more about what the "paddling" will do to him rather than worry about him missing a vacation to the beach.