The reason that the DS414slim is so small is that it uses 2.5" drives (as commonly found in laptops) rather than 3.5" ones (as commonly found in desktop computers). It can hold four of them and they can be configured as JBOD, RAID 0, RAID 1 or RAID 5. At present, drives of up to 1.5TB capacity can be utilised, giving a total storage capacity of 6TB.
The price of the DS414slim is expected to be around £250/US $350 for a bare unit.