My DS1 (4) can be left anywhere, with anyone. On holiday, we dumped him in a casual holiday camp in the next house, full of local kids who knew each other already. Which was almost entirely in French, a language he doesn't speak. (Two of the teenagers running the camp had good English, though.)
And that was the highlight of the whole trip, for him. I'm used to being able to dump him anywhere, particularly if there are other kids there.
Are other people's four-year-olds like this?