Split PCI-E x16 to 4x PCI-E x4?

I see there are PCI-E riser cards which can convert x1 or x4 to x16. I am not sure how useful they are for multiple GPUs because of the bandwidth limitation, but probably they are ok for GPU intensive tasks like bitcoin mining. What I want to do is the opposite. I'd like to convert an x16 slot into muliple x4 slots to be able to use multiple ethernet cards. I understand that buying a switch might be a better solution than converting a PC into a switch, I am just curious if this is possible...