How about this Shuttle micro tower?
PRO:
12.3"x12.3"x7.5" so pretty small
four drive bays (two will need adaptors)
PCI-E 16x and PCI for expansion
CON:
only two SATA150 ports and one EIDE
network only 10/100
THOUGHTS:
Perhaps it might fit the bill. Pair it with the cheapest A64 (for power management). May be able to add a gigabit ethernet card and SATA card using expansion slots.
A mini ITX possibility
Looks like an overgrown Shuttle cube, but can hold 4 drives.
Alternately go micro ATX and live with the size of a mini tower. Benefits will be wider choice in motherboards and power supplies, and cheaper than most mini ITX solutions (though the Shuttle I listed is cheapest). For instance
this case has four HDD spots, but can take a total of 8 drives in a 14.3"x15.4"x7.2" size. Pair that with a mATX board that has four SATA 3.0 ports and onboard GBe, toss in a more efficient PSU and you're golden!