Does anyone know if Halion 4 is compatable with cubase pro 9? Have you used it?

Does anyone know whether Halion 4 and mr816csx are compatable with Cubase 9 Pro??

As long as you have the 64 bit version I don’t see why not. I have used Halion One, Five and Six with Cubase 9 without issue.

It will work of course. I strongly recommend to upgrade to Halion 6 if you can though because it’s just plain better in several aspects, and it’s much closer to being bug free.