For renders sufficiently demanding the bottleneck will be the processing power not the pci-e interface bandwidth. X4 is perfectly fine.
see the following topics from @Todd_Takehana:
/t/tested-cycles-gpu-rendering-with-pcie-x1-risers-and-rtx-optix-support/
/t/tested-1300-8-gpu-blender-cycles-rendering-does-cycles-scale/