Here is something I recently found out. I have tired this and it works.

You can use a PCIe x1 card, or a PCIe x4 card in a PCIe x16 slot. They work fine. So if you think you are short of PCI-e slots, you may not be. A shorter PCIe card works fine in a longer slot.

On PCIe X1, X4 and X16 slots, the first 18 pins are all wired the same, so as long as the PCI-Express card fits in the PCI-Express slot, it should work.

This is not the case for the legacy PCI (32-bit, non-express) slots, that are not PCI-Express. Those are not compatable with PCIe cards.

Here is the PCI-Express page on Wikipedia showing pinouts, etc.
PCI Express - Wikipedia, the free encyclopedia

YouTube Video showing PCIe X1 card in a PCIe X16 slot.
x1, x4 pci Cards Fit Into x16 pci-e Slot - YouTube

Here are links to discussions on this subject at Tom's Hardware.

"Can I install a PCI-e x1 Wireless network card in a PCI-e x16 slot?"