Per Dell:
4600 PowerEdge Server
Seven PCI slots located on the system board. PCI slots 2 through 7 are hot-pluggable 64-bit, 100- or 66-MHz slots. Slots 2 through 7 also support PCI-X functionality. Slot 1 is a 32-bit, 33-MHz slot that can be used for the optional DRAC III or other PCI-compatible options.
Expansion Bus
Bus type
PCI or PCI-X
Expansion slots
six 64-bit, 66-MHz/33-MHz slots (PCI) or
six 64-bit, 66-MHz/100-MHz slots (PCI-X)
one 32-bit, 33-MHz slot
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Try slot 1 if you haven't tried it yet.
If its available, check the status of the slot in BIOS. (I don't remember if that model has that option)
I have a 2600 next to me right now, but its not powered up so I cannot check it.
Dan