If you can find drivers for your scanner, I'd go with a windows box running 64-bit Windows XP. 4x5 scans get very large - easily half a gig in size and you need all the memory you can handle. 32-bit OS's just don't provide enough memory capability. I would trade processor speed for RAM and disk space - say 8 GB RAM and at least 500GB hard disk. For PC's you can get an external SATA drive with a SATA connector that will be much faster that firewire or USB.
Again, this is all dependent on your scanner having 64-bit drivers available.