Google brought me here and I thought I'd make a few corrections.
It is not a 4 pin like fans use. it is a legacy 4 pin like floppy drives use. It is for the PCI to PCIe converter chip because the Xonar D1 and probably your D2 as well are/is actually a native legacy PCI card and in order to get it to work on modern mother boards it uses the PCI to PCIe converter.
The regular PCI versions have less glitch/issues than the PCIe version, if one has problems to begin with.(windows side people use unofficial drivers to fix this)