Z790 Pro RS with 256 GB RAM

For RAM compatibility it's always best to check the compatibility page on the RAM's
webpage. The support lists from motherboard manufacturers almost never get updated.
It makes sense, once the product is launched it doesn't make financial sense to
keep testing and verifying every single RAM module that becomes available. With
as many board models as there are it would take an incredible amount of labor
to do. RAM manufacturers are far more numerous so it's easier to let them test
a selection of boards with each of their RAM models than to test potentially
hundreds of new RAM modules every few months on dozens of board models for each
platform, not to mention older platforms.

A quick look at GSkill's 256gb kits shows no kits listed with support for intel
platforms at all, from any brand (ASUS, MSI, ASRock etc).

For now I would avoid 256gb kits on your system, at least until more are certified
for Intel's XMP architecture. Every kit I found only lists AMD EXPO support, a
good indicator that it won't work on Intel systems or it would list EXPO AND XMP
support.

here is a 2x64gb kit from GSkill with XMP support

As you can see here the 4x64gb kits only support EXPO.

Your best bet is to either wait and use a smaller kit for now or check Redit/google
for anyone else who has successfully run 4x64gb kits on a Z790 board.