Assuming that they actually agreed to work together (an alliance that wouldn't last very long due to certain members' selfish tendencies), and assuming that they are all providing all the resources they have, then they would become extremely powerful. Probably strong enough to capture most multiverses, although each universe has multiversal threats that would stand in if necessary. For example, it is unlikely any of these villains could defeat Dark Dimension Vader if they attempted to conquer his universe.

If, however, they are not attacking the multiverse and instead only the universe itself, they would probably be able to kill hundreds of thousands of planets.

Well yes this would never happen but this is theoretical

The absolute carnage all 4 of them would commit is unfathomable with the whole army Palpatine has! Darkseid is just immensely powerful same as Doom.

This would probably the mark the end of the Star Wars & Monsterverse universes, as powerful as the Star Wars characters can be Darkseid and Dr. Doom take things to another level. I do think the combined forces of Marvel & DC could still win, but Darkseid and Dr. Doom alone could create so much destruction, you through Ghidorah and Palpatine in there as well and it would be a multiversal level war.

It's implied that Ghidorah had destroyed thousands of planets prior to coming to Earth, and Darkseid can destroy an entire universe by just being there, so I think they could destroy a lot.