Well, it's the same model, just that the 9000 is for the US and the UK.

It has something to do with Hollywood Standard (and 4.4.4 parameters).
There is something about it on the internet:
https://heimkino-klang.de/shop-2...

My DP-UB9004 runs flawlessly with self-produced UHD 4K ( and 4.4.4 ) videos.

Is the successor to the 9000... improved hardware, ThX certified etc.

That's a very good question, to which I still haven't found an answer! I assume it's a non-EU model but that's just a guess.
Apart from that it's an excellent player, I'm also looking for a simple way to unzone it for US DVDs if you have a tip I'd be interested!