The memory is hidden from OS or what? If it is not and OS is able see it, it has to cut some RAM as well as some VRAM since the system needs something to run. Usually it cuts the RAM first, but it probably just cannot adress 4GB VRAM and 0 GB RAM because it won't even start.
The x86-32 systems simply adresses up to 4GB and the rest is unavailable and could not be used as long as it is not hidden from the OS somehow. Or at least this is what I saw every time with 256 or 512MB cards. But I personally did not have card with higher amount of memory in hands
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