Should we admit that world wars don't need to involve superpowers - or at least only superpowers? The term wasn't that much applicable before end of WWII.

So here we can argue that in WWIV a non-superpower fights - Ukraine, on its territory, a superpower - USA, merely - but with principal results - supporting Ukraine, and the rest of the West. We may not call it a proxy war - I agree, it's a rather poor comparison - but for WWIV term it is another matter.

World wars don't need to involve superpowers, but they need to involve large part of the world. Russian invasion on Ukraine doesn't, and it's unlikely to escalate - Russia can't, because they have neither people, hardware, or allies, and the defending countries don't have a reason to.