I am writing a comedy horror series. The first book was set in a basement, the second book was set in a Ski-Resort and the third book was set in Italy, meaning that each book’s setting was progressively bigger and more exciting than the last. However, I do not know where to go next, as every other setting I envision for the 4th book seems like a downgrade from the international setting of the last book. Where do I go next?
This is a somewhat tricky question as I don't know the general plot of your book, but this is what I would recommend:
You could possibly escalate it to a global/different world/alternate reality, but, does that make sense for your book? You said your book was comedy horror, which usually doesn't go to that big of a scale. You could however like Ceramicino0b pointed out, take your story to several countries, for example, it could be based all throughout Europe.
On the contrary, I personally would, not consider escalating the setting at all.
You could write a basement scene that is 10x more exciting than a ski-resort and you could write a ski-resort scene that is 10x more exciting than an Italy scene. You could even write a scene in a basement of a ski-resort in Italy!
So, don't think of it as growing the setting, think of it as raising the stakes.
For example, you could have a detective solving a murder mystery in a ski-resort and the case could attract global attention, thus raising the stakes and further escalating your story.
Apply this strategy to your actual plot and you can avoid having to take your story to an international level.
The setting should adjust with your characters.
The setting is only a part of the story.
Its main purpose is to serve character development and the plot.
So, instead of adjusting your characters to work with the setting by escalating the scene, think about adjusting your setting to work with the characters.
Raise the stakes, not the setting.