Inspiron 3668, 4G decoding

My desktop is dell Inspiron 3668, I am using pcie riser to add 4th GPU to the system and encounter "BlinitializeLibrary failed 0xc00000bb error" when I googled lot of people are talking about 4g "decoding" should be enabled. But I don't see this option in BIOS system. 

Am I missing something here or my specific Motherboard won't support 4GPU's?

Thank you!