Most games I tried at 3840x2160 res with HLSL enabled (default settings) used no more than 500MB or VRAM. "RC de Go" used 600MB of VRAM. I beleive a vidcard with 1.5GB or 2GB of VRAM should be fine for MAME at 4K resolution. I couldn't make OpenGL or BGFX effects work though. For some reason, enabling the OpenGL setting is not working for me.