Synology has added a more powerful four-bay 2.5" only NAS.
The DS411slim is a compact (5" tall x 4" wide) four-bay NAS that uses only 2.5 drives.
It's basically a DS409slim with a 1.6 GHz Marvell Kirkwood CPU and 256 MB of RAM. Port complement includes one 10/100/1000 Ethernet, two USB 2.0 and one eSATA.
It will run DiskStation Manager 3.0 when it's released in "mid-February". Pricing has not been set, but Synology said the DS411slim "will be more affordable than the DS409slim (currently around $370)".