Hi:
While I don't have that model, on all of my HP business desktops with the black and white PCIe x16 looking slots, the black one is PCIe x16, and the white one is PCIe x4.
The illustration labels for the slots are correct. Black is x16 and white is x4.
Here is the service manual from one of my PC's.
http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c04224518-1.pdf
Ssee chapter 6, page 78 where everything is properly labeled.