I can't imagine any other country joining them though. It's always been the case that China has expected the Anglo world and their Japanese sex slave to contribute to platforms in the AO and possibly even combat platforms. If even I knew this then I'm sure China came to that conclusion a long time ago..
Other than those four there really doesn't seem to be much potential. Sure, maybe countries like Germany or Poland or Italy or even South Korea will give some kind of tacit support. Maybe allow aerial refueling, overflight, use of bases for logistics, intelligence sharing, etc. but I highly doubt they'd ever get directly involved outside of a support role.
You could have in some scenarios India or Vietnam or Philippines joining in. Mostly as base use or even limited operations (Vietnam using its subs, India trying to seize some territory). But I think even those countries know how absolutely awful it would be if China wasn't permanently crippled in that same conflict. Imagine having to live with China for the rest of Human history after performing such a sneak attack. Therefore I highly doubt even those SCS nations or India would get involved in such a direct manner. Support for the Anglos and their puppet slave state in Japan? Sure, but I doubt they get involved in direct combat operations. I'm sure China would actually wish they did though, it would give China the excuse to seize all the SCS islands Vietnam controls and all the disputed territory with India.
As for the matter of South Korea... not in a billion years. Again a support role is possible after severe US pressure, but can you imagine them actually becoming directly involved? China basically shares a land border with them through North Korea. Nothing would suit China better than to have the South Koreans join. It would justify an invasion of the South and a permanent eradication of the American presence on the mainland, possibly for all time.