The Morex Venus 669 can apparantly "support up to 4 x 3.5in drives or 2x 5.25in optical drives, but not at the same time". Sounds like it may do the job, but it is rather small and I'd expect it to be tight and hot.

On a slight tangent, the Buffalo Terastation is a fair priced NAS device which comes with 4 disks. It runs Linux and can easily be hacked to run some stuff on the side. There is a Wiki. Not sure how well it would work for whatever you're doing though.