I came across the RAID products from Raidweb.com. I don't know if RaidWeb is some sort of OEM or just re-badges equipment, but their SIVY3187 model is a 4-drive unit (using SATA drives). External interface is eSATA only.

The SIVY3187 has hot-swappable drive bays and in addition to typical RAID 0, 1, and 5, it also does 0+1 (mirrored across striped pairs) and RAID 5 with hotspares.

RaidWeb's SIVY3187 enclosure

Pricing isn't crazy expensive, but a bit out of my range for home use. The bare chassis alone is $1295. Fully populated with 4x750GB drives (from the vendor) would cost $2495. For $1295 I think there's plenty of other products that can do just as a good a job for about 1/3 the price.