I understand the thread is quite dated, but the topic is still highly relevant so I am digging it up: There is even a PCIE1x to 4 individual USB 3.0 controllers out there, but it is "made-to-order" and it really puts the freaking into freaking expensive: https://www.ioi.com.tw/products/proddetail.aspx?CatID=106&DeviceID=3021&HostID=2035&ProdID=1060142. You can check ebay and these cards go for 400-600USD a piece.
I would expect the U3X4-PCIE4XE111 (https://www.ioi.com.tw/products/proddetail.aspx?CatID=106&DeviceID=3035&HostID=2035&ProdID=1060179) to work just fine in a PCIE 1x slot, as long as you physically modify the slot (just cut away the plastic at the rear end of the slot that prevents the card from getting inserted) ... So for 200 bucks less, I'd rather bring out the Dremel and buy the non-gimped version.