I'm not sure there's really a need for a version 4 of the RaspberryJAMMA hardware. RPi4 already works with any existing version, with the exception of the HDMI size change the only thing that doesn't line up right is the reset header (which can still be used).
I do already sell version 3 RJ boards with micro HDMI DACs as an option (no price difference). A simple HDMI to micro HDMI adaptor will convert any boards with full size HDMI to work on Pi4 too.
Vulcan should be good once it's working properly, but I'm not sure what if anything it will add. It should certainly improve performance in some use cases but it will vary from emulator to emulator.
Questions about the RaspberryJAMMA adapter.