NAS devices come in all shapes and sizes nowadays. QNAP is of course one of the bigger names known for its high-end products, and it now has the new QNAP TS-420, a 4-bay NAS. It's perhaps not as deluxe as some of the top of the line models, but it still features a metal body and sturdy hard disk trays. You don't get a display or a way of locking the drives, however.
It has plenty to offer in other ways, especially in terms of connectors in the back. Having a look at those is a good way of telling how high-end a NAS device is. You should of course also ask yourself if you will actually use every single one of those, or if a more basic model would be sufficient. If you can afford it, the more, the merrier of course.
Two out of the three USB ports are 3.0, and there are two gigabit network ports. For adding storage there are two eSATA ports. In the front you'll find another USB 2.0 that can be used in combination with the synchronization button. Of course USB 3.0 would have been preferable. And despite the fact that the QNAP TS-420 can't be called compact, the power supply is external, and also not the smallest.
The hardware inside the NAS consists of a 1.6 GHz Marvell Kirkwood processor and 512 MB of RAM. That's a bit less powerful than the higher positioned QNAP TS-421 which has a 2.0 GHz processor and twice the amount of memory, along with a display. That's a practical feature to have, but there's about a £100 price difference as well between the TS-420 and TS-421.