From Ars Technica:

Ars takes a quick look at Synology's DS409slim, a full featured NAS device that packs four laptop hard drives into a svelte package. There are some tradeoffs in slimming down, but they're not quite what you may think.
Honestly, space is my biggest concern and not storage. with 4 500 gig drives doing some RAID ... I think i could survive.