Questions about the Heroes 4 map editor.

And also, if the map settings allow for items from the upcoming expansion, will they drop, and what artifact group do they belong to?

When initializing random artifacts on the map, artifacts from the upcoming expansion will not drop. Regarding treasure chests, I don't know; we'll check now. They most likely belong to the Relic artifact type or some separate artifact group.

Added 12 minutes later
I checked, they don't drop in the Dragon City. Just like other Relic class artifacts. Instead, random artifacts of other classes drop, even if they are prohibited on the map.

Added 8 minutes later
I don't know about the probabilities of artifact drops in treasure chests. But I'm almost certain that each treasure chest has probabilities for dropping a specific class of artifacts, but not each specific artifact. That is, for example, in the Dragon City, the Soulreaver, Emerald Bow, and Book of Life will drop with the same probability because they all belong to the Major class. As for the probabilities themselves, unfortunately, I can't say anything. If you find anything, please post it here. I'm also interested.