I'm trying to start a Dedicated Fullscreen D3D9 Window using a big resolution. With 3x4k resolution (11520x2160) it works only if I create the device with one back buffer (D3DPRESENT_PARAMETERS::BackBufferCount = 1). If I try to create 4x4k (15360x2160) it never works unless I create the window in Windowed mode (D3DPRESENT_PARAMETERS::Windowed = true).
I've attached a sample code that will create a FullScreen window and a EDID for the 4k montior I'm using so you can test with a 4x4k Eyefinity. The code will work if you use two GPUs, one for the 4x4k such as a W8100 and a main GPU. If I create a Windowed borderless device it works fine in all cases.