It should work, pci-e was designed to allow lower speed cards to run in higher speed slots as electrically they are identical down the slot with added length of the card contacts simply adding more bandwidth. At least that is how I've always understood it.
Hi all, I have a Poweredge T310 running server 2008 R2 and I need to add a fibre HBA card. We have a PCI-E LPE1150-E x4 HBA card that we were intending to use for this purpose.  [URL] Upon inspecting the server, both X8 slots are being used. I cant use the X1 slots. The only available slot is the x16 (circled below). Can I use an X4 card in an X16 slot? Will the be an issue or would it be better to run this in an x8 slot? We currently have a scsi card in the extra x8 slot for backup purposes.  [IMAGE] 
Fibre HBA PCI X4 in and X16 slot Poweredge T310