Generally TTX units have some kind of whitelisting/blacklisting on the video card that can be used.
What we have seen with the TTX3 is that trying some of the firmwares available for the cards we were trying to swap at some point worked. But we were not able to identify a clear rule and it was more or less "at some point of the firmware worked", we even had some card which worked only by using a firmware from another vendor like you would buy a MSI card and the video card finally worked by using a PNY firmware.
I assume this will be mostly the same story with the TTX4, I would go with MSI as iirc this is the prefered video card vendor Taito worked with.