From ATTO's website...
"Backward compatible with PCI-based systems, preserving investment in legacy systems
133 MHz PCI-X provides up to 1 GB/second data transfers, allowing full Ultra320 performance
Fully backward compatible with 66 MHz and 33 MHz PCI specifications.
Fully compatible with 64-bit and 32-bit PCI busses"
If you have a UL3D why do you need a UL4D? Using Media100i, the UL3D is more than sufficient.