Apple is still working on fixing the iPadOS 18 issue for the iPad Pro M4 models, and I’m guessing they’ll release it as soon as next week, along with the iOS 18.0.1 update.
If you’re eager to upgrade to iPadOS 18 and don’t want to return your iPad just because it can’t update yet, you might want to try installing the iPadOS 18.1 Beta and check out the Apple Intelligence features early.